Still with VirtualBox, afaik one has to setup a lot, e.g. adding an ISO to an iso storage and configuring the VM. With KVM one can just create a nice shell alias and boot e.g. Fedora Test release live images with one command:
alias kvm_iso="qemu-kvm -boot d -k de -m 1024 -usbdevice tablet -cdrom"
kvm_iso F13-Alpha-i686-Live.iso
And when I kill the VM, nothing is left behind (log files, stale configs, ..).