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 allocated resources:
xm list - list all VMs on the hypervisor by identifier
Use xm top to see current stats for the VM:
xm top - show vital stats for all currently running VMs
See the xm man page for more information.