How can I read a paramater passed in a URL to an XML file?

XML isn't a language you execute. So something else is processing your XML, right? Its that something else that can provide you with the values in a number of ways. One way (if you are generating dynamic XML) is to provide the request as XML elements. Another way is to pass the parameters to the XSL processor, which can make them available via extension functions. See here for examples of using Apache Xalan with extensions.