Thank you for pointing to the mechanisms in use here!

If you download the live Xfce ISO and transfer it to a USB stick, you get (at least I get) an ordinary Xfce user interface on boot. If you click on the terminal icon (may be done as many times as needed), you get a terminal where you can run fdisk

sudo fdisk -l

The listing will tell you how the live Devuan has mapped the drives it has found, and it should be easy to figure out which disk to choose for install. You may post the listing and ask here, if you are not sure. Sda is the drive, sda1, sda2 etc are partitions

Here is the fdisk listing of the sda on the machine I write this (with Devuan 6.1.1 running :-))
It's an 1 TB Samsung SSD SATA disk with 'oldfashioned' MBR partitioning. Fdisk today handles both this and 'modern' GPT partitioning.

Disk /dev/sda: 931.51 GiB, 1000204886016 bytes, 1953525168 sectors
Disk model: Samsung SSD 870 
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xed595e8a

Device     Boot      Start        End    Sectors   Size Id Type
/dev/sda1  *          2048   41945087   41943040    20G  c W95 FAT32 (LBA)
/dev/sda2         41945088   83888127   41943040    20G 83 Linux
/dev/sda3         83888128  230688767  146800640    70G 83 Linux
/dev/sda4        230688768 1953525167 1722836400 821.5G  5 Extended
/dev/sda5        230690816 1677725695 1447034880   690G 83 Linux
/dev/sda6       1677727744 1814042623  136314880    65G 83 Linux
/dev/sda7       1814044672 1953525167  139480496  66.5G 83 Linux

Sure.
Still, there have been uncovered two bugs here:
1. Lacking noautologin parameter makes live image only run in failsafe mode. Extremely simple to fix.
2. When user 'user' is defined, noautologin does not work, 'user' is logged in. Whatever the Debian live people write in their manuals, this is wrong behavior.
None of these bugs should be left for ordinary users to fix.
But, of course doing nothing about it is also an option.

The user 'devuan' is not what the Debian live build procedures expect. I just created the default Debian user  'user'  in my PMI install. Works fine. With autologin: User 'user' is logged in.
But I think it may be quite good practice to disable autologin. I just activated it to demonstrate that the problem is there.

I tried first with that, no luck.
And now I'm happily running 'failsafe' without nomodeset ;-)

No, that's not at all obvious, because any emulation has to be output through the host hardware. It may be important in some situations, but through nearly 30 years of running OS in emulators, I have never needed to dig deeply into the emulated hardware to solve problems.
The VM's emulated hardware may not even be that well defined.

Furthermore, I responded to a posted native inxi-G listing with my own.  You can obtain the corresponding listing from the VM by installing and running inxi there. To me, the information is of little help, as the real problem seems to lie elsewhere:
Behavior problems under kvm and running natively were identical, as far as I could see.
When failsafe mode worked perfectly both under emulation and - when tried - running natively, what is the probable relevance of digging into the emulated hardware?

Quite a lot. You may check it in the grub.cfg on the ISO.  Maybe vga=normal is an important parameter.

I wonder if recreating Debian 13.3.0 live with the SDK version they have used, and then adapting the procedure to the Devuan 6.X context could be a viable way to proceed? From the end products, there does not seem to be large differences from 11.X to 13.X. The same kernel parameter setup works, for example.

Please tell which of the differences are clearly relevant for the issue we are discussing.
The important context here, is that nobody seems to get the graphics on the ISO running under QEMU/KVM in any simple way, and we wonder why.
While failsafe mode works perfectly for me, as I have reported. I have also pointed out that two different pc's, with different displays, behave the same for me.

Thanks a lot for the responses! Useful!
Still, no explanation why failsafe option and root login works like a charm in the simplest setup:

sudo kvm -m 2048 devuan_excalibur_6.1.1_amd64_desktop-live.iso &

Now I have tried it on two Probooks, 440/450 i5 24GB G5&G6, with Debian 11.2 LXDE and 13.3 Lxqt. Exactly the same results. And trying to run from RAM ends in kernel panic.
Is 'failsafe' kind of kernel parameter so I can use it in the GRUB kernel stanza?
The nonfree-ISO behaved the same way for me: Everything works in failsafe mode, otherwise black screen.

Small update:
I could even run

apt-get update 

and

apt install rxvt-unicode.

From ISO under kvm. Install went fine, wherever that landed...
Could I then run

urxvt &

and get a working terminal window? Yes.

I haven't tried with Devuan yet, but I have handled Debian 13.3.0 with complete success. Both on USB sticks and external SSD disks.
I have found legacy grub to be the most reliable companion, and I haven't needed any special tools.
I use three partitions (minimally)

1. An ext2 partition for booting
2. An ext4 partition for compressed live images
3. An ext4 partition for persistent storage
All these partitions can hold several OS versions, and one version (I have used Debian 11.2.0 as the basis for the last 4 years) may go through several iterations (typically 3-6) of compressed live images during a lifetime of 2-5 years.

This can be done on a 32+ GB USB 3.X stick with 30+ MB/s writing times. Using a slower stick is hardly worthwhile. A SATA disk or an m2-disk in an external case is simpler in daily use, but hardly necessary. 64GB is a practical mimimum, 256 GB kind of sweet spot. If you think you need more, you haven't really tried to be space efficient. If you are sure you need less, you have probably not lived through the full OS version upgrade blues.

Using live versions with persistent storage are, traditionally, called Poor Man's Installs (PMI). There is a tradition going back (at least) to Klaus Knopper and KNOPPIX. He made a 32-bit version-hybrid Debian derivative, therefore very hard to upgrade, but very, very useful for a couple of years.

In the Debian context, a Poor Man's Install is based on 4 files:

1. vmlinuz - the kernel. Which does not have to be changed during the lifetime of an OS version, but of course, YMMV.
2. initrd.gz - often Busybox-based init code. Hackable, I once made a pure 64-bits Knoppix version starting with a modified initrd.
3. filesystem.squashfs - the compressed live image. Which you may recreate as needed. I just expanded the original Debian 3.2 GB Lxqt image to 8.8 GB, including ca 250 R-cran packages plus GIMP, VLC, WXMaxima etc. If you think you have to grow the image way above 10 GB, consider Docker for some of the large components.
4. persistence. A loop-mounted overlay file system, typically 2-20 GB. No, you don't have to restrict yourself to small programs when you work based on a live system. Oracle XE, Postgresql and even Axiom (math theorem proving etc) work just fine.

In the booting partition, booting files are organized in directories by OS releases. For example. I have  deb910 (LXDE) , deb1120 (LXDE), deb1330Qt (LXQt) to mention a few. Upgrading kernel/initrd is of course possible here, and may be best practice. But I have never experienced an urgent need for that, and I think it is safer to start a new OS version install than upgrading the kernel. This partition is mostly read-only.

The live image partition is read-only in everyday use. But it may be updated several times.

The persistence partition holds 1-2 persistence files for each OS version, union mounted with the compressed image: The one in current use and eventually the previous. When it starts to get full, it is time for 'remastering', simply copying the whole union file system into a directory, squashfs-compressing it into a new live image and setting up a new persistence file.

Surprisingly, old legacy grub is just fine for booting in this context, and it even works on a USB stick set up with three (or more) Linux partitions. Here is the booting stanza for Lxqt Debian 13.3.0 with one 'remastering'. This is on an internal disk, using a stick would be similar.

title Debian 13.30 64 bits lxqt live sda3 v1 persistence sda6 
      root(hd0,0) 
      kernel  (hd0,1)/boot/deb1330Qt/vmlinuz boot=live config quiet splash initrd=(hd0,1)/boot/deb1330Qt/initrd.img  bootfrom=/dev/sda3 live-media-path=deb1330/live1 persistence persistence-path=deb1330Qts/pers1  keyboard-layouts=no
      initrd (hd0,1)/boot/deb1330Qt/initrd.img