how to display xml content ?

Xiang, have you already tested with the ThingWorx Marketplace Extension called Universal Converter Extension ?

You can't display XML content in the composer. You can view it either by creating a mashup and binding the result to a widget such as the textbox, or via the rest API.

If you want to display XML content on the composer, set Result type to String, and do var result = ""+yourXMLObject;

if you want to test the service, just set the base type in "Inpu/output" to info table. From snippet call GetXML(), set the url in double quotes.

that should work