http://community.citrix.com/cdn/xs/sdks

Xen-API
C            libxenserver
C#.NET        XenServer.NET
Java        XenServerJava
PowerShell    XenServerPSSnapIn
Python        XenAPI.py
OCaml         (available but not released)

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http://docs.vmd.citrix.com/XenServer/5.5.0/1.0/en_gb/api/docs/html/browser.html

Citrix XenServer Management API