In /usr/src/xen-sources, do:

wget http://gentoo-xen-kernel.googlecode.com/files/xen-patches-2.6.31-7.tar.bz2
wget http://aur.archlinux.org/packages/kernel26-xen-dom0/kernel26-xen-dom0/microcode_core-xen.patch
wget ftp://ftp.kernel.org/pub/linux/linux-2.6.31.5.tar.bz2

tar xjvf linux-2.6.31.5.tar.bz2
tar xjvf xen-patches-2.6.31-7.tar.bz2
cp patch.sh linux-2.6.31.5
cd linux-2.6.31.5
sh patch.sh

patch -p1 -s -i ../microcode_core-xen.patch || return 1

cd ..
rm 6*   // clean patch dir

XEN kernel is ready to be configured.

patch.sh file content:

#! /bin/sh

for i in `ls ../6*.patch* | sort `
do
patch -p1 -s -i $i
if [ $? = 0 ]; then
echo "$i applied"
else
echo "Error processing $i"
exit 1
fi
done