As of September 2010, vhost is not included in any released tarballs, so you need the git version.
VhostNet provides better latency (10% less than e1000 on my system) and greater throughput (8x the normal virtio, around 7~8 Gigabits/sec here) for network.
Obtaining the code
You need the at least qemu-kvm-0.13.0 (http://sourceforge.net/projects/kvm/files/qemu-kvm/)
On the host you need a kernel with CONFIG_VHOST_NET=y and in the guest you need a kernel with CONFIG_PCI_MSI=y
example command line
To start a guest with vhost:
- kvm -netdev type=tap,id=guest0,script=kvm-ifup,vhost=on -device virtio-net-pci,netdev=guest0,mac=ma:c:a:dd:re:ss ..other options..
Some DHCP clients have problems with this implementation, more details here.