EDIT (2015-11-14): Please note that this post was made in 2011. At that time, this was the only solution. Comments suggest there are better ways now. I have not investigated. Current project involves moving some KVM VMs from file storage to block storage. This is basically converting the .qcow2 files to LVM LVs. It was surprisingly simple. First, we need to get an idea of how much space our VM uses.
Being consistent with my preferences in life, this is not a simple task. At least, figuring it out was not a plug-and-play activity we are all used to in the VMWare world of virtualization. I want: A KVM Windows Server 2008 R2 guest installed on an LVM logical volume utilizing the VirtIO drivers for storage. The LVM portion was pretty straight forward. Create an LVM out of the free space in my PV.
Recently I had a couple of harddrives die on me and I replaced them with a pair of WD RE4 2TB drives. This brought my physical capacity up from 1.5T. Of course, I now want to take advantage of that extra space. My system is an Ubuntu 10.04 Server with the two drives in a RAID1 array, a LVM partition scheme on top of that with only 1 volume group. The system boots off the array.