I just had to resize a virtual disk and it was harder than I thought; writing down what I did so it’s easy to find …

1. shutdown VM

   qm shutdown 108

2. resize disk image

   qemu-img resize vm-108-disk-1.qcow2 +20G

3. attach pmagic CD; boot to pmagic (using proxmox management web interface)
4. extend physical partition

   pvdisplay; lvdisplay; 
   fdisk -u=sectors /dev/sda
   delete partition 2 (remember starting sector) (‘d’, 2)
   create new partition (n) starting at same sector as before; end is defaulted

5. extend pv

   pvresize –v /dev/sda2

6. extend lv

   lvresize –L +20G VolGroup/lv_root

7. resize filesystem

   e2fsck –f /dev/sda2
   resize2fs /dev/sda2

8. detach pmagic cdrom
9. start VM

   qm start 108

Good instructions for working with Windows (NTFS) disk images is here: http://cauldrondevelopment.com/blog/2009/02/26/resize-qemu-ntfs-image/