Please remove kvm-source
The package in question, kvm-source, is another leftover
from old/retired/transitioned package kvm. It is not
in squeeze anymore, and it was a mistake to include it
in lenny too, since it breaks even lenny's kernel.
Note that the original kvm package is already removed
from bpo, and replaced with a dummy transitional pkg.
This package contains source for kvm kernel modules,
using module-assistant. The thing is that upstream
kernel includes these modules for a long time already,
and now (even in lenny) the modules in kernel are
more recent than available in kvm-source package, and
current kvm does not work with the old kernel module
from kvm-source (hence kvm-source breaks current
kernel), while current kernel works just fine even
with very old kvm userspace.
Please remove kvm-source from backports.
I'm also filing a bugreport against kernel to add
Breaks: for kvm-source, so that users wont break
their kernels (it has already happened unfortunately).