just check for the /proc/xen/capabilities file.
Sounds like the OP is looking to distinguish xen guests. The file above is also present on hosts, or any system running a xen kernel I'd guess. Can this file also be used to reliably distinguish between guests and non-guests? On the few systems I've got to check against, the capabilities file contains: "control_d" on hosts, while on all (pv) guests, the file is empty.
Also I don't know how reliable, this is, or will remain, but xen guests are the only systems I've got where 'lspci' returns nothing (successfully).
-Ed