Xen API (or xapi) is a management stack that configures and controls Xen-enabled hosts and resource pools, and co-ordinates resources within the pool. Xapi exposes the Xen API interface for many languages and is a component of the Xen Cloud Platform (XCP) project. Xen API is written mostly in Ocaml 3.12.
Xen Cloud Platform (XCP) is an open source enterprise-ready server virtualization and cloud computing platform. XCP builds on the Xen Hypervisor; and provides network and storage support, and management tools in a single, tested installable image.
XCP runs many guest operating systems, including Linux and Microsoft Windows.
You can usually find the developers hanging out in #xen-api on freenode.