http://support.citrix.com/article/CTX121748

Xen and the Art of Virtualization:
http://www.cl.cam.ac.uk/research/srg/netos/papers/2003-xensosp.pdf

Safe Hardware Access with the Xen Virtual Machine Monitor:
http://www.cl.cam.ac.uk/~akw27/papers/safehw-oasis04-final.pdf

Facilitating the Development of Soft Devices:
http://www.cl.cam.ac.uk/~akw27/papers/usenix-softdevices.pdf