From KVM

Paravirtualized Block Device

Host side

run kvm with command-line: -drive file=<path_to_your_raw_qemu_img>,if=virtio

you can also specify boot=on in the command-line: -drive file=<path_to_your_raw_qemu_img>,if=virtio,boot=on

Guest side - Linux

sudo modprobe virtio_blk

to have virtio_blk driver, you need to upgrade the kernel to higher than 2.6.25.

refer to Virtio

Guest side - Windows

You can find Windows drivers at Downloads page.