The concept of an XML database, as far as I know, is largely theoretical. And it's not entirely clearly defined, either.
*deep breath, this is going to take a minute*
XML DB's imply storing data in a tree-based format. MySQL is not itself entirely compatible with this format, as it is a table-based db.
That being said, the main interpretation of an XML db is as a table-based database with scripts to store XML documents.
I myself have tinkered with the concept of reorganizing a tree-based document structure into a table of nodes. You'd be surprised to see what I came up with.
I do believe I've seen references to XML databases which are not based on tables, probably at SourceForge.
As I see it, an object-oriented database (like ZOPE) would be much more compatible for storing XML nodes, as a tree-based format and an object-oriented data format are very closely related indeed.
Note this is all opinion, and not an expert one at that. You may have to do some research.