What commands can I use on a XEN hypervisor if I want to see which VMs are currently running there and the resources they are currently using?
Use list to see the currently running VMs and the allocated resources:
#xm list - list all the VMs on the hypervisor by identifier
Use xm top to see the current stats for the VM:
#xm top - shows vital stats for all currently running VMs
See the xm/xl/virsh man page for more information.