Below is a quick solution to increase the size of a XEN-PV disk that works fine on all our nodes (paths and names may vary depending on your control panel setup, here Solus is being assumed):
- shut down the VM
- lvextend /dev/VOLUMEGROUP/vmID.img -L +[INTEGER]G
- e2fsck -f /dev/VOLUMEGROUP/vmID_img
- resize2fs /dev/VOLUMEGROUP/vmID_img
- boot the VM
This should work for any standard xen-pv template (we use the ones from Stacklet).