Converting VirtualBox VM (VDI format) to KVM (qcow2 format):

#list available VirtualBox HDDs and note Capacity and Location
VBoxManage list hdds

#Convert VDI to RAW image
VBoxManage clonehd --format RAW /home/wuff/VirtualBoxVMs/Win10/Win10.vdi win10.img

#Convert RAW image to qcow2 image
qemu-img convert -f raw win10.img -O qcow2 win10.qcow2

https://www.phoronix.com/forums/forum/hardware/processors-memory/1069922-linux-kvm-continues-offering-much-better-performance-than-virtualbox

https://wiki.archlinux.org/index.php/QEMU

https://linuxhint.com/android_qemu_play_3d_games_linux/