What I understood from your question is that you have a VM box that acts as a Hub and you have Xen Desktop.
To trigger tests on Xen Desktop from your VM box is more of a topic of networking.
1.) You need to make sure that both are on the same network.
2.) If they both are part of different networks, you need to make sure that from your VM box, you can access the Xen desktop.
If any of the above condition is true, you can have your VM Box has a hub and your Xen Desktops as nodes connecting to the VM Box as a hub. You will just have to provide URL to VM Box to RemoteWebDriver class.
When you trigger tests, the hub will take care of redirecting tests to Xen Desktops. You do not have to host a separate hub on the Xen Desktop.
Hope that's the setup you are looking for.