From Ewan Mellor on Citrix Community Blog:
One of the most common requests from XenServer customers is for a web interface, so that they can manage their VMs from a browser. I’m pleased to say that I’ve found one!
It’s called xvp, and has been developed by Colin Dean at Durham University in the UK. There are four components:
•   xvpweb: A web front end for XenServer, running on Apache/PHP.
•   xvpviewer: A Java applet for accessing XenServer consoles (used by xvpweb, of course).
•   xvp: A server-side proxy for XenServer consoles, so that you can use ordinary VNC clients.
•   xvpdiscover: A tool that queries a XenServer pool, and writes the appropriate configuration files for xvp and xvpweb.
The whole thing is open source (GPL v2). It uses libxenserver, XML-RPC for PHP, and parts of TightVNC. It’s been in development since May 2009, and is now on version 1.2.4.
Project home page: http://www.dur.ac.uk/c.c.dean/xvp/