Comment 38 for bug 292588

(not tested)
If you want to use VirtualBox as your default VT solution and if you don't want to uninstall kvm, try to not load the kvm modules at startup by adding a new file in /etc/modprobe.d/kvm-blacklist, containing these directives:
blacklist kvm
blacklist kvm_intel

That should work after the next reboot. After that, in case you need to run KVM just for one time, you need to load the kvm modules thanks to "modprobe kvm && modprobe kvm_intel" before lauching your KVM machine.

What is the best behavior for the installation of the package in order to have VBox running out of the box? I don't know. For example, exim is removed when you install postfix (Conflicts: mail-transport-agent), but we can maybe imagine a thing like an update-alternative machanism (?)