What is the extent of radar coverage provided by XM?

It depends on which radar product is used. The composite radar is limited to the lower 48 states, with some coverage extending beyond the northern and southern borders:

Image 1.  XM composite radar coverage.

The lowest-tilt radar extends coverage further north into Canada, and also provides coverage for Puerto Rico and islands south of it, extending to Grenada and Barbados:


 Image 2.  XM lowest-tilt radar coverage.