In a similar way to needing to find the state of VMs that were on a XenServer storage volume to "do other stuff" with I came across the need to understand the disk usage of XenServer VMs on an NFS volume. Yes: it is the exciting life I lead.
If you consider the XenServer Command Reference, there isn't a direct command for this action (because Virtual Machines have Virtual Block Devices which in turn reference the Virtual Disk Images .. which are what are stored on the disk)
What XenCentre will do is show you the *expected* size of the VM i.e. what you said the maximum capacity would be. Lovely. If you're on an LVM volume that is what it is, but if you've an NFS volume then you can have more sensible conversations about capacity vs usage - but you can't readily see that information.
What I wanted to know was:
a) the storage use of the NFS volume
b) to understand the disk usage per VM
Without a direct xe command, I had to script a solution.