Hi Oguz,
thank you for your reply.
All that happened after I rebooted the server after the upgrade.
[root@pve1 ~]$ ls -al /dev/kvm
ls: cannot access '/dev/kvm': No such file or directory
I did another reboot and now everything is working.
[root@pve1 ~]$ ls -al /dev/kvm
crw-rw---- 1 root kvm...