Page Valet 4 is based on mod_validator. This now supports multiple parsers, including currently OpenSP (the "reference" SGML Parser implementation) and Xerces, a validating XML parser.
When you use Page Valet, you can express your preference by selecting a parser from the menu. Valet will check your selection against the content-type of the document to be validated, and will normally honour your selection, although it may override you if you try to select a parser unsuitable for the document type.
If you don't know what parser to use, select the "Default" option, and the system will automatically select a parser suitable for the document type indicated in your HTTP headers. The validator report will tell you what parser has been used.
This is the reference implementation for SGML, and is the validator engine used by other online validators including the W3C and WDG validators. It will validate HTML and other SGML document types, and (with a few limitations) also DTD-based XML. However, it lacks support for XML Schema and Namespaces.
This service uses a slightly-customised version of OpenSP 1.5.
Xerces is the Apache Group's XML parser, and was the validator engine for the Site Valet XML validator until that was merged into Page Valet. It has no support for HTML (except XHTML), but its XML support is more complete than OpenSP, and it can deal with XML Schema and Namespaces.
This services uses Xerces-C 2.2