Comment # 6 on bug 939544 from Dominique Leuenberger

just retried this:

zypper in -t pattern xen_server
=> kernel-xen is not marked for installation

zypper info -t pattern xen_server --recommends
=> kernel-xen is a recommended package for xen_server

zypper install kernel=xen
=> it can be triggered for installation without issues, so there is generally
no reason why zypper would not install this recommended package (it does
trigger others)

The only thing I could find is a weakremove(kernel-xen) < 3.11 in
openSUSE-release,  but as this is versions, I sure hope it does not impact here