XQuery based native XML databases

Deciding when an XQuery-based native XML database is better than an SQL database: Link

A plethora of information about Native XML Databases can be found at www.rpbourret.com

Of all the available NXDs Berkeley DB XML seems to be the better choice.

Berkeley DB XML is a library, and as such is linked into your application just as any other library would be, it does not incur the overhead of client/server communication. You interact with Berkeley DB XML using one of the supported language APIs. The primary one is C++, as the product is written in C++. Java, Python, Perl, PHP, and TCL are all supported API languages.