Comment # 1 on bug 948482 from Dominique Leuenberger

The same can be observed with:

> sudo zypper in -t pattern xen_server
Loading repository data...
Reading installed packages...
Resolving package dependencies...

The following 5 NEW packages are going to be installed:
  libspice-controller0 patterns-openSUSE-xen_server virt-viewer xen
xen-doc-html

The following NEW pattern is going to be installed:
  xen_server

The following 3 recommended packages were automatically selected:
  virt-viewer xen xen-doc-html

5 new packages to install.
Overall download size: 10.3 MiB. Already cached: 0 B. After the operation,
additional 35.8 MiB will be used.
Continue? [y/n/? shows all options] (y): 


The pattern is going to be installed, but kernel-xen is not in the list of
packages to add.


The pattern does list kernel-xen:
> zypper info -t pattern xen_server
Loading repository data...
Reading installed packages...


Information for pattern xen_server:
-----------------------------------
Repository: openSUSE-Tumbleweed-Oss
Name: xen_server
Version: 20150828-2.1
Arch: x86_64
Vendor: openSUSE
Installed: No
Visible to User: Yes
Summary: Xen Virtual Machine Host Server
Description: 
  Software to set up a server for configuring, managing, and monitoring virtual
machines on a single physical machine.
Contents:

S | Name                         | Type    | Dependency
--+------------------------------+---------+-----------
i | bridge-utils                 | package |           
  | kernel-xen                   | package |           
  | libvirt-daemon-xen           | package |           
i | patterns-openSUSE-base       | package |           
  | patterns-openSUSE-xen_server | package |           
  | virt-manager                 | package |           
  | virt-viewer                  | package |           
  | vm-install                   | package |           
  | xen                          | package |           
  | xen-doc-html                 | package |           
i | xen-libs                     | package |           
  | xen-tools                    | package |           
i | xterm                        | package |           
i | yast2-vm                     | package |           

installing kernel-xen manually does not cause any resolution conflicts:

> sudo zypper in kernel-xen
Loading repository data...
Reading installed packages...
Resolving package dependencies...

The following NEW package is going to be installed:
  kernel-xen

1 new package to install.
Overall download size: 45.0 MiB. Already cached: 0 B. After the operation,
additional 211.9 MiB will be used.


So, this sounds like libzypp/libsolv is not giving a complete solution to the
problem at hand (or something pretends that this kernel-xen is not needed for
the pattern)