Because the Makefile directly checks for python 2.2, 2.3 or 2.4 it fails on 2.5. Quick and dirty patch here fixes it, though it should probably be changed to a comparison.
Created attachment 98095 [details, diff] Lets xen-tools detect python 2.5
Upstream changeset is (if I got the branch right) http://xenbits.staging.xensource.com/xen-3.0.3-testing.hg?cs=b7f763a8a7ef
Fixed in app-emulation/xen-tools-3.0.2-r4, thanks.