I can think of two interpretations of your problems.

(1) You want have an XML document which you want to pass to the client.
Solution: just pass it to the client. Presumably, it's already valid XML, or
else that web server you have is broken.

(2) Your client is expecting a XML encoded response which semantically
represents a string. It just so happens that coincidentally, this content is
XML data.
Solution: Escape the relevant characters.

For example, if you want to return the String which represents the following
XML document:

<XmlDocument>Hello World!</XmlDocument>

then escape the string as follows:

"&lt;XmlDocument&gt;Hello World!&lt;/XmlDocument&gt;"

and then put that string within your XML response:

<XmlResponse type="string">&lt;XmlDocument&gt;Hello
World!&lt;/XmlDocument&gt;</XmlResponse>

and then your problem reduces to (1) which was already solved.

   - Oliver