It depends on what platform and with what options XBMC is built with. Since the configure script required at least 2.4 and 3.x is generally not compatible, that indeed leaves 2.4 - 2.7. By default, you'll get the one installed as a -dev package on the compiling machine, which will most likely be the default one of your distribution.
2.1.0 is just the version number of the Python add-on, and unrelated to the Python installation itself.