Found two different sources, which converge only in the fact that the best disc format - raw. In the first link that says Device Virtio is the best and that without the cache faster than most, but in the second link tests show that the best IDE, but if you use the virtio, it is better to put writeback cache
http://pve.proxmox.com/wiki/Performance_Tweaks
http://www.linux-kvm.org/page/Qcow2#Growing_images_.28virtio.29
http://pve.proxmox.com/wiki/Performance_Tweaks
http://www.linux-kvm.org/page/Qcow2#Growing_images_.28virtio.29