Side note: the Tiger, Leopard, and Snow Leopard VMs in this screen shot were running OS X Server to comply with Apple's pre-Lion license restriction prohibiting virtualization of non-Server versions of the OS. I have no idea which hypervisors, if any, enforce this restriction (nor if old versions of OS X Server are still available for purchase directly from Apple and/or available for non-production use through the Apple Developer Program as they once were).