libxml (http://xmlsoft.org/) is distributed with most distros for xml parsing, but it is covered under the MIT License (http://www.opensource.org/licenses/mit-license.html) which is similar to GPL but it does not mandate the availability of the source code.

You can find a list of xml parsers at http://www.xml.com/pub/rg/XML_Parsers you will have to follow the links to determine the licenses used for each.