New KVM guest system disk not properly expanded

I’m creating KVM guests (using ubuntu 12.04 on a CentOS 6.3 host) and choosing a disk size of 50-75GB depending on the machine I’m spinning up.

The disk images are in an LVM group, and I see no problem with that side of things.

Whilst the virtual disk available to the guest is resizing to the correct 50-75GB size, the guest root filesystem remains just 2GB.

Ideally I’d like this to be expanded automatically along with the disk itself, but failing that is there a way to increase the size of the / partition else safely relocate the /var /usr and /home directories to new partitions on the guest’s virtual disk?

Any ideas would be welcome.