Netvault does not support backing up a XEN hypervisor server itself.
NetVault can backup/protect XEN Virtual Machines (VMs) running in the XEN environment.
Xen and KVM are supported by running a NetVault client (and plug-in, if desired) within the guest OS.
In addition, from the NetVault Compatibility Guide. Please see page 8 of the guide for addition support notes and comments. NetVault
Backup Compatibility Guide for Hardware and Software
NetVault Backup Server and Client components are supported when running in a virtual machine (VM) as long as
NetVault Backup is running in a hypervisor guest OS that is officially supported by NetVault Backup. Supported
hypervisor programs are: KVM, Hyper-V®, Oracle® VM server, Virtual Iron®, VMware® ESXi™, and Xen®.