pg_xnode is an extension (module) which adds several data types to the PostgreSQL database server, allowing it to store XML documents, XML (tree) nodes, and XPath expressions. Unlike the in-core implementation of XML, pg_xnode does not require repeated parsing of XML data, and thus should be more efficient. In addition, the extension has functions to add/remove nodes to/from existing XML documents, and templates to create XML documents from tables.
Sirix is a versioned tree-based storage system. It provides Snapshot Isolation through MVCC and is best suited for flash-based storage. The tree encoding is suited for heavy update operations. It provides diffing algorithms either to import diffs between currently shredded (imported) XML documents to update a tree structure or to get the differences between versions of a tree structure. A GUI provides several visualizations for comparing imported tree structures.
jOOX stands for Java Object Oriented XML. It is a simple wrapper for the org.w3c.dom package to allow for fluent XML document creation and manipulation where DOM is required but too verbose. jOOX only wraps the underlying document and can be used to enhance DOM, not as an alternative. It is inspired by jOOQ and jQuery.