Maven is a project building & documentation tool.
XP is usually associated with lack of documentation. (IE. "we re introducing XP - we're not doing any documentation anymore!"). As relieving as it may be to the developer, it's not the correct approach. With XP tries to avoid creating useless documentation - the documentation that no one will care to read, and documentation that will become outdated quickly during project development.
In XP the source code is the primary documenation - it alwas stays up to date. Any documentation that can be derived from the sources will also stay up to date, provided that you regenerate it periodically.
Maven integrates a number of tools that generate useful reports based on the code and it's tests and combines them into a coherent HTML document hierarchy.