The major difference between HTML and XML is that HTML describes presentation and XML describes content of website. HTML document is human readable and XML is for both human and Robot(machine readable).
XML also has some rules regarding content that HTML does not: some characters must be translated into named(
) or numeric ampersand-pound-number-semicolon entities (sorry, can't show you , daniweb converts them even in code mode!). I wrote a program to do this translation. https://sourceforge.net/projects/uritoxml They also have different headers.
XML can be transformed into HTML, XHTML, or any other markup language using XSLT.