You can use your own phones that are compatible with Verizon network. However, as of this time, Xfinity Mobile does not let you order only the SIM but you must order a phone with the SIM for the new line. XM reps said BYOD would be available soon but they didn't say when. So far, the LG V20 US996 and the Moto E4 (VZ Prepaid version) that I bought from somewhere else can run my XM SIM with no problems.
Anything that works on the Verizon network (including Nexus devices that are not carrier-specific) should work, they will just show "Verizon Wireless" as the carrier instead of "Xfinity Mobile" (which appears to be baked into the firmware of Android phones sold through Xfinity Mobile).
But it is not supported. In other words when everything works it is great, but when you have a problem and need technical support then you are out of luck. While I wish they supported BYOD, it is not unreasonable for the company to limit the number of supported devices. After all this is a new service and the support line is already overloaded.