I don't think you can use the JAXP service mechanism to search for an XSD 1.1 processor. Load Saxon or Xerces in the normal way, and then enable XSD 1.1 processing. For Saxon this is done using
SchemaFactory.setProperty("http://saxon.sf.net/feature/xsd-version", "1.1")
UPDATE 2021-09-04 In recent Saxon releases, XSD 1.1 is the default, and no special property needs to be set to enable it.