Thanks to all,
I did explain, that there is an absolute strong need to setup a working system as it was on my running Proliant DL380-G5. All suggestions to start with a new system did not fit the (my) needs. The main problem was, on my Fujitsu Esprimo 900E90E testserver (i5 and 8GB ram and 2 x 128GB SSD) runs my KVM sample on a 42.3 host with 4 different (sample-) guests 12.3, 13.2, 42,3 and 15.1. And all guests run super. Installation from a fast net-repository was less than 3 minutes for each guest !!! = 15 minutes for all 4 sample guests !!!
That means, finally “it works”. I did not find out, why the XEN solution did not (or shoud not) work in my datacenter. We are talking about a “dedicated root server” from “Hetzner” or “myloc webtropia” (MSI board with i7 cpu and 32GB).
At the end I found out, that their special inhouse DHCP server does not fit the needs. Whatever is wrong with their DHCP configuration, the decline any responsebility. Installing 42.3 and 15.1 as XEN oder KVM guests works fine with some 3 minute timeout (to get a temp ipv4) compared with my inhous test (0,1 second to display that DHCP ipv4).
Comparing whith my test environment in my laboratory, it could work with their support or help, but they dont want. The main featue of XEN and KVM ist installing and running any guest. Virtbox does this very well with all windows versions from WIN98 on. But Virtbox ist not my solution for a remote server in production environment.
So at the end I did evaluate how a VM ist started and installed on a 42.3 host as a guest und I did replace the VM root with a (on my test-server) preinstalled 12.3. — Funny is, with the 12.3 rescue CD you cannot fix any difference, you need a 42.2 net ISO to start with and do not go to rescue line but to options line and in that menue of very well unknown second page - start a simple menue entry word “start linux” and the installed 12.3 did start - wounderfull without any problems, just modyfying the ip number and gateway with yast and manually an additionally pointopoint entry – and it runs. (since June 1 - the large hifimuseum pages are back online)
That means, you need to evaluate all the tricks, to pass the rocks. (and there are many rocks on the road) - The next step is, to install the 42.3 (or 15.1 this did not run seemless in january) on my test-server with the XEN environment and try to install the same 4 guests - with my DHCP server and from my remote repos.
and I will report, if that works. — it should work.
Another tip :
I did delete all xen modules on another 42.3 test-server (with guests down) and did install the KVM modules on that running system (all with yast2) and the virt-manager could start all old XEN guests - now under KVM.
That means, the virtlib platform - looking upwards to the started guests - is layer compatible !!!
a very good experience
thanks again for reading my texts - I hope, it will help other readers with similar problems