KVM
I have been converting some of my old VirtualBox images to KVM, which is the virtualization standard in Linux.

Its really easy, although, if you run into booting issues of the converted image, the last tip here will save you some time.

To convert from VirtualBox to KVM, look for your VDI images.

qemu-img convert -f vdi -O qcow2 [VBOX-IMAGE.vdi] [KVM-IMAGE.qcow2]

e.g. qemu-img convert -f vdi -O qcow2 FreeBSD-7.2.vdi FreeBSD-7.2.qcow2

You can now import the converted image using virt-manager or the CLI tool virt-install.

In Virtual Machine Manager, go to Disk 1 >> Advanced options and change ‘Storage format’ to qcow2.

This prevents boot errors like ‘boot failed: not a bootable disk’ when you launch the VM.