Are you trying to get the UUID from the virtual machine, from the hypervisor, or both?
If it’s from the virtual machine, you should be able to retrieve it from the ‘dmi’ or ‘smbios’ inspectors.
If it’s from the hypervisor, it depends. I haven’t seen a BigFix Proxy Agent for Xen, not to say that there isn’t one. You may be able to file an RFE or engage Professional Services to create one. If there’s some method or command line to retrieve the info from another system that runs the BES Client we may have some othet suggestions.