Thats no problem. Use either the DOM or a simplexml to convert it into actual usable xml in php. I'm a fan of DOM myself, so that would be constructed as simple as:
$xml = DOMDocument::loadXML($yourCURLResponse);
And simple xml is as simple as:
$xml = new SimpleXML($yourCURLResponse);
Note that if this is simply to push back to JS, than you needn't at all push this into a XML type. Simply add a header:
before any output, and print the results of the curl response. That will now be served as an XML document instead of a text/plain. Only use dom/simplexml if you plan on actually manipulating the xml in some fashion.