VB on KVM is still VB and nothing else. It uses VB guest additions, not virtio and Co, etc.

There are only 3

Depends on whether you include both type-1 and type-2 hypervisors, and whether your universe is limited to only those that run on GNU/Linux.

That's not what you did, you took a comment out of context to imply that the proprietary virtualbox hypervisor must be "bad" because they're adding KVM as an additional option, then used that as as an excuse to sling mud at "big evil corporation" for taking too long to include your favourite alternative.

My response is "supporting both is good, because they serve different needs. The more options we have the merrier." I have no loyalty to either technology.

If you want to insist Qemu/KVM is universally "better", I invite you to try running Visopsys or PC-DOS with full emm386, or moving your VMs between different host operating systems (which is  much easier with a type-2 hypervisor like VB).
Like I said, use-cases... One solution does not fit all of them.

Multiple options are good.

There are only 3. (without XEN which is not linux but rather xenux, without bosch etc). The third one is bought by corporation worth than Oracle or similar and closed source. The second one can be used simultaneously with the first one when it use hypervisor from the first one (KVM) = take the best from 2 worlds.

This is exactly what I`ve wrote already.

P.S. As always, everybody is free to use whatever s/he likes and everybody is free to give an advise based on h.. own experience.

Oracle decided to move towards KVM

You mean "finally release the KVM backend they've been working on for a while". Nobody nowhere stated it will replace their own hypervisor, or even become the default.

Recall Oracle didn't develop VirtualBox, Sun Microsystems did. What remains of that team is likely quite small, VB is not even remotely a priority for Oracle as a whole and big corporations do love their silos.

Are you perhaps implying that Linux KVM has no security vulnerabilities? A rather easy CVE search suggests otherwise...

...Except when it's not. DOS and OS/2 guests spring to mind. Multiple options are good.

They could not do it before because of the lack of resources. Oracle .... But other people could

Vulnerability exploited over a network https://www.oracle.com/security-alerts/ … pendixOVIR
KVM is better.

just start the network permanently with

virsh net-autostart default 

What does this

sudo usermod -a -G operator <user>

do?

check your /etc/libvirt/libvirtd.conf/etc/libvirt/libvirtd.conf

It’s worth to have there

unix_sock_group = "libvirt" 
unix_sock_rw_perms = "0770"

in

/etc/libvirt/qemu.conf

set

user = "user" 
group = "kvm"

Do not forget to add user to aall this groups: kvm,  libvirt, *qemu*

then rc-update add libvirtd
and
rc-service libvirtd start (if you are using openrc, if sysvinit or runit then find appropriate commands to enable service)

After reboot you'll have started networ "default", running libvirt and user wil be granted access to use virt-manager and virt-viewer (I find virt-viewer more comfortable to run VMs, then virt-manager which is more suitable for creating them).

Also note that uefi can be broken in virt-manager. last time I have to copy uefi bios from somwhere and I don't know if thhe situation have been changed since.

If  you are looking for alternatives you can try  https://github.com/quickemu-project/quickemu  and GUI for it https://github.com/quickemu-project/quickgui

The gui will download and install guest for you. You can also copy uefi bios from this install in the cse you need it for virt-manager and you can run all these guests in parallel (virt-manager/viewer and quickemu).

More of that you can use https://github.com/ivan-hc/VirtualBox-appimage - this is an appimage of virtualbox that use KVM as hypervisor, not Oracle's core component!  It can be run in parallel with qemu, aqemu, quickemu, virt-manager etc.

Have fun.

sudo virsh net-list --all
 Name      State      Autostart   Persistent
----------------------------------------------
 default   inactive   yes         yes

ip a

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host proto kernel_lo 
       valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 52:54:00:61:61:72 brd ff:ff:ff:ff:ff:ff
    inet 192.168.122.163/24 brd 192.168.122.255 scope global dynamic noprefixroute eth0
       valid_lft 3385sec preferred_lft 3385sec
    inet6 fe80::6702:1847:b353:47ad/64 scope link noprefixroute 
       valid_lft forever preferred_lft forever

i have noticed it did not show a virbr0 entry .