XenIntro - introduction to Xen : Implementation details and Design
XenEventChannels - the use of event channels in Xen, including common channels
XenMemoryManagement - page table updates, sharing pages between domains etc.
XenBusDriver - a reference for those writing Xen aware Linux kernel modules, especially `split' drivers.
XenSegments - the use of segments, including the use of negative offsets
VirtualFramebuffer - design/implementation discussion for a Xen Virtual Framebuffer
XenSplitDrivers - Architecture for Split Drivers Within Xen
NB: Some of the above articles were either not written by Xen developers or are considerably out of date. If you are capable of updating/correcting this information, please do. Also, while user contributions are greatly appreciated, if you're not sure if you understand a particular subsystem fully before doing a write-up, please use an appropriate disclaimer.
Articles and Papers
A complete list of Xen related papers and publications is available at http://www.xen.org/community/xenpapers.html