1. Stop the VirtualBox machine.
2. Convert VDI Image to RAW Disk Format
vboxmanage clonehd --format RAW /home/user/VirtualBox_VMs/Debian_vm.vdi /home/user/VirtualBox_VMs/Debian_vm.img
3. Copy RAW image to KVM server
rsync -av --progress /home/user/VirtualBox_VMs/Debian_vm.img user@kvmserver:/home/kvm/Debian_vm.img
4. Convert RAW Image Disk Format to KVM Format
qemu-img convert -f raw Debian_vm.img -O qcow2 Debian_vm.qcow2
Źródło: https://www.tecmint.com/migrate-virtualbox-vms-into-kvm-vms/