Google for "virtualbox to kvm migration." It is pretty easy, depending on your specific needs.
There are caveats. Don't migrate desktops looking for better graphics performance. Virtualbox is the best Free/OSS solution for desktop-on-desktop VMs today. KVM is great for server-on-server virtualization. I've migrated Xen, Vbox and ESXi VMs to KVM. These were for servers, except 1 office-productivity-type desktop, which I connect to using NX from the same room or 12 timezones away.
Always know the virtual hardware on the source VM and on the target VM. If these are easier today if you select virtIO for network, disks and ICH for everything else. Remove things that do not work on the next virtual hardware ... like guest additions.
I don't have any detailed notes about vbox to KVM migrations, so it must have been trivial. Sorry.
http://blog.jdpfu.com/2012/10/23/remote-desktops-rock is all that I have - the
vboxmanage command was used.
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