On the Dom0

resize the LV for the DomU and reboot

$ lvresize +5G /dev/DomU/pv2
$ xm shutdown mk-scripts-1
$ xm start mk-scripts-1

On the DomU

Resize the PV

$ pvresize /dev/xvdc (may require --setphysicalvolumesize size)

extra space will then be available in the VG
$ vgdisplay -s

And can be added to any LV that needs it.

$ lvresize +5G DomU-VG1/home