This namespace, http://www.w3.org/2001/05/PSVInfosetExtension, is used for XML serialization of post-schema-validation (PSV) infosets. It contains only the PSV additions to the basic (pre-schema-validation) namespace http://www.w3.org/2001/05/XMLInfoset.

There is an XML schema for this namespace at http://www.w3.org/2001/05/PSVInfosetExtension.xsd. It is not intended to be used directly, but is imported by http://www.w3.org/2001/05/PSVInfoset.xsd to produce a schema covering a complete PSV infoset.

For more information, see these notes.