Virt-manager uses QEMU and I think it is pretty good, and much faster than Virtualbox. If you wish to use a desktop though, you should configure the video card of VM manually, its performance is not that good, but if you really want to use desktop VM, I suggest VMWare, I found the interaction much faster, but Virtualbox is pretty good to.

In short, QEMU with KVM is way faster, for servers, non-gui interactions.