I wrote a blog about this topic, here is the url
sleebing's blog: Install BT4-rc2 on KVM+virtio

Install BT4-rc2 on KVM+virtio
Recently, I tried to migrate my virtual machines KVM. I tried to install BT4-rc2 on KVM as soon as I got it. I first set the disk type IDE, but I found it was very slow to install BT4-rc2 in the virtual disk. So I changed the virtual disk's type to virtio. I finally could run it after a few hours struggle.

Here is a brief summary.
* Boot from BT4-rc2.iso.
* Compile and load virtio modules.
* Install BT4-rc2.
* Make new initrd file
* Copy virtio modules and initrd file to the target disk.

** Compile and load virtio modules
virtio modules are not included in BT4-rc2's kernel. So the installation program could not find the virtio disk /dev/vda. For supporting virtio disk, I compiled and loaded virtio modules.
> cd /usr/src/linux

--- Config virtio Modules ---
> make menuconfig
Mark all virtio modules, they are in
->Virtualization (basic modules of virtio)
->Device Drivers->Block Drivers (virtio disk driver)
->Device Drivers->Net Drivers (virtio net driver)

--- Compile basic virtio modules ---
> make SUBDIRS=drivers/virtio modules
> mkdir drivers/net/virtio

--- Compile virtio net module ---
> mkdir drivers/net/virtio
> cat drivers/net/Makefile|grep virt> drivers/net/virtio/Makefile
> cp drivers/net/virtio_net.c drivers/net/virtio
> make SUBDIRS=drivers/net/virtio modules

--- Compile virtio block module ---
> mkdir dirvers/block/virtio
> cat drivers/block/Makefile |grep virt> drivers/block/virtio/Makefile
> cp drivers/block/virtio_blk.c drivers/net/virtio
> make SUBDIRS=drivers/block/virtio modules

--- Copy and load modules ---
> mkdir /lib/modules/
> cp drivers/virtio/*.ko /lib/modules/
> cp drivers/net/virtio/*.ko /lib/modules/
> cp drivers/block/virtio/*.ko /lib/modules/
> depmod -ae
> modprobe virtio_blk
> modprobe virtio_pci

** Make new initrd file
> mkinitramfs -o /tmp/initrd.img-

** Copy to target
Copy virtio modules and initrd file to target disk after BT4-rc2 has been installed.
> mount /dev/vda1 /mnt
Replace vda1 by the partition in which BT4-rc2 is installed.
> mv /mnt/boot/initrd.img- /mnt/boot/initrd.img-
> cp /tmp/initrd.img- /mnt/boot/
> mkdir /mnt/lib/modules/
> cd /usr/src/linux
> cp drivers/virtio/*.ko /mnt/lib/modules/
> cp drivers/net/virtio/*.ko /mnt/lib/modules/
> cp drivers/block/virtio/*.ko /mnt/lib/modules/