apt-get install linux-image-2.6.18-4-xen-686
Reading package lists... Done
Building dependency tree... Done
Package linux-image-2.6.18-4-xen-686 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package linux-image-2.6.18-4-xen-686 has no installation candidate

dpkg --get-selections \* | egrep '^linux-image.*xen.*'
linux-image-2.6-xen-686                         purge
linux-image-2.6-xen-vserver-686                 purge
linux-image-2.6.18-4-xen-686                    purge
linux-image-2.6.18-4-xen-vserver-686            purge
linux-image-2.6.20-1-xen-686                    purge
linux-image-xen-686                             purge
linux-image-xen-vserver-686                     purge