Xen Visualization is hardware based so would be harder to setup, you'd probably need physical access to the server to do it. OpenVZ is software based and very easy to setup over SSH. I don't believe you can run both Xen and OpenVZ on one node at the same time?
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No, I'm talking about if software can control ever client/vps creation, with this order: WHMCS-->Master SolusVM-->Slave SolusVM.
Now i'm running WHMCS at node1, Master SolusVM at node2, and if i go for a new XEN node, i'll running Slave SolusVM at node3.
I don't know if WHMCS can handle node3 via node2 Master SolusVM or directly.