You've put your Windows system in /Devices/Server. That's very non-specific and I think expects a Linux system (maybe). You should try putting them in /Devices/Server/Windows if you want to use SNMP Informant (on the Windows Servers), and really ought to consider installing a zenpack for either SNMP or WMI monitoring from the community. Figure out what you want to do and I can help more.

First, new graphs take 3-4 collection cycles to generate graphs. This often takes 15 minutes or so. For Zenpacks, you may want to look at the following:

docs/DOC-3410 - Dell Manage Agent

Make sure to meet the pre-requesites/requirements for zenpacks though.

docs/DOC-3386 - Windows SNMP

docs/DOC-3500 - Windows WMI Performance

Note, the two above duplicate functionality, so you'd choose if you prefer to collect that data via WMI or via SNMP and use one of the zenpacks.

You could try libvirt for your xenserver:

docs/DOC-4687

Note this requires you rename the zenpack to install on v3 - see the last comment there. You could also just monitor the VMs as servers individually, and I might recommend that as well. Then just add them to the appropriate device classes when adding them...