You may run into something like this while updating or installing your Xen platform:

>>> Emerging (1 of 3) app-emulation/xen-tools-4.1.0-r1
* xen-4.1.0.tar.gz RMD160 SHA1 SHA256 size ;-) ...                                   [ ok ]
* Package:    app-emulation/xen-tools-4.1.0-r1
* Repository: gentoo
* Maintainer: [email protected]
* USE:        acm elibc_glibc flask kernel_linux pygrub userland_GNU x86
* FEATURES:   sandbox
>>> Unpacking source...
>>> Unpacking xen-4.1.0.tar.gz to /var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/work
>>> Source unpacked in /var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/work
>>> Preparing source in /var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/work/xen-4.1.0 ...
* Applying xen-tools-3.4.0-network-bridge-broadcast.patch ...                        [ ok ]
* Applying xen-tools-3.3.0-nostrip.patch ...                                         [ ok ]
>>> Source prepared.
>>> Configuring source in /var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/work/xen-4.1.0 ...
>>> Source configured.
>>> Compiling source in /var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/work/xen-4.1.0 ...
make -j2 -C tools
make: Entering directory `/var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/work/xen-4.1.0/tools'
make[1]: Entering directory `/var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/work/xen-4.1.0/tools'
make -C check all
make[2]: Entering directory `/var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/work/xen-4.1.0/tools/check'
PYTHON=python LIBXENAPI_BINDINGS=n ACM_SECURITY=y ./chk build
Xen CHECK-BUILD  Mon Apr 11 16:19:03 EDT 2011
Checking check_crypto_lib: OK
Checking check_curl: unused, OK
Checking check_openssl_devel: OK
Checking check_python:
*** check_python FAILED: need python version >= 2.2
Checking check_python_devel: OK
Checking check_uuid_devel: OK
Checking check_xgettext: OK
Checking check_xml2: OK
Checking check_zlib_devel: OK
Checking check_zlib_lib: OK
make[2]: *** [check-build] Error 1
make[2]: Leaving directory `/var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/work/xen-4.1.0/tools/check'
make[1]: *** [subdir-all-check] Error 2
make[1]: Leaving directory `/var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/work/xen-4.1.0/tools'
make: *** [subdirs-all] Error 2
make: Leaving directory `/var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/work/xen-4.1.0/tools'
emake failed
* ERROR: app-emulation/xen-tools-4.1.0-r1 failed (compile phase):
*   compile failed
*
* Call stack:
*     ebuild.sh, line  56:  Called src_compile
*   environment, line 4879:  Called die
* The specific snippet of code:
*       emake -C tools ${myopt} || die "compile failed";
*
* If you need support, post the output of 'emerge --info =app-emulation/xen-tools-4.1.0-r1',
* the complete build log and the output of 'emerge -pqv =app-emulation/xen-tools-4.1.0-r1'.
* The complete build log is located at '/var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/temp/build.log'.
* The ebuild environment file is located at '/var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/temp/environment'.
* S: '/var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/work/xen-4.1.0'
 
>>> Failed to emerge app-emulation/xen-tools-4.1.0-r1, Log file:
 
>>>  '/var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/temp/build.log'
 
* Messages for package app-emulation/xen-tools-4.1.0-r1:
 
* ERROR: app-emulation/xen-tools-4.1.0-r1 failed (compile phase):
*   compile failed
*
* Call stack:
*     ebuild.sh, line  56:  Called src_compile
*   environment, line 4879:  Called die
* The specific snippet of code:
*       emake -C tools ${myopt} || die "compile failed";
*
* If you need support, post the output of 'emerge --info =app-emulation/xen-tools-4.1.0-r1',
* the complete build log and the output of 'emerge -pqv =app-emulation/xen-tools-4.1.0-r1'.
* The complete build log is located at '/var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/temp/build.log'.
* The ebuild environment file is located at '/var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/temp/environment'.
* S: '/var/tmp/portage/app-emulation/xen-tools-4.1.0-r1/work/xen-4.1.0'

The problem is due to Xen's check-build not recognizing your Python 3x installation as "greater than or equal to 2.2." You can resolve this by temporarily switching your Python profile:

# eselect python list
Available Python interpreters:
  [1]   python2.4
  [2]   python2.5
  [3]   python2.6
  [4]   python3.1 *
# eselect python set 3

It might be advisable to run python-updater and any other updates you have before compiling xen-tools with python2x; once you switch back to the latest version of python xen-tools will be picked up by python-updater and fail to compile.

For more information please see my bug report at http://bugs.gentoo.org/show_bug.cgi?id=363299