From SemanticLab

Paravirtualization

• You have to boot your guest with a kernel supporting virtio (>=2.6.25).
• Change the harddisk settings from /dev/hdaX to /dev/vdaX in the following files (see virtio devices for more information).

  /etc/fstab
  /boot/grub/device.map
  /boot/grub/menu.lst