You can not have VirtualBox and KVM running on the same host at once.
If you want to do Android App development on Linux (which uses KVM for running the Emulator) then you have a problem 😀
KVM seems to be very competitive with a few drawbacks.
VirtualBox is optimized for desktop usage, so it has a few advantages over virt-manager:
- copy-paste works
- resizing the resolution according to window size works
- mostly 3D acceleration works
it seems to be a major hazzle to get Clipboard working with KVM? http://www.linux-kvm.org/page/SPICE