Howtoforge posted a tutorial about creating VirtualBox backups with LVM snapshots

If you use LVM volumes for your VirtualBox VMs, you can create backups and clones of a running VM without shutting it down. This tutorial shows just that: using LVM snapshots to create backups and clones of running VirtualBox VMs without downtime.

  VirtualBox: Creating Backups Clones Of Running Virtual Machines (No Downtime) With LVM Snapshots