These are some packages used for XML processing in Emacs or XEmacs:

Here is an article by BrianGillan (IBM) on Getting and installing Emacs, Adding PSGML for SGML and XML modes, Editing a document with PSGML, and Using SP or OpenSP for SGML and XML validation…

From the BobDuCharme book SGML CD, here is a longish (99 pages) chapter on using Emacs to edit SGML/XML files using PSGML. Also contains Polish and Russian translations.