Virtualbox is a very well known KVM, which is the virtualization standard in Linux.
In our case we had some vms on windows using virtualbox and we wanted to migrate those vms to our new linux server.
Step 1: Convert from VirtualBox to KVM
Search for all the files with .vdi extension and convert those files to qcow2 format using the qemu-img:
qemu-img convert -f vdi -O qcow2 [VBOX-IMAGE.vdi] [KVM-IMAGE.qcow2]