How to return xml data from webservice to jquery ajax call from a webform

This is my JQuery AJAX call. This is inside the document.ready() function. This is supposedly the one that will read the xml data returned by the webmethod in my webservice:

            $.ajax({
                type: "POST",
                url: "http://tempuri.org/NewsletterList.asmx/HelloWorld",
                contentType: "application/xml; charset=utf-8",
                dataType: "xml",
                success: function (xml) {
                    $(xml).find('Newsletter').each(function () {
                        var title = $(this).find('Title').text();
                        var created = $(this).find('Created').text();
                        AddOption(title);
                        alert('Ywes');
                    });
                },
                error: function (msg, m2, m3) {
                    alert(m2);
                }
            });

This is my webmethod call in my webservice. I am able create xml sucess fully but i am finding difficulty in returning xml back to ajax call.

    [WebService(Namespace = "http://tempuri.org/")]
    .
    .
    [WebMethod]
    [ScriptMethod(ResponseFormat = ResponseFormat.Xml)]
    public XmlDocument HelloWorld()
    {
        //Instantiate model object
        nl = new Newsletter();

        //Initiate XML stuff
        StringBuilder sb = new StringBuilder();
        XmlWriterSettings settings = new XmlWriterSettings();
        XmlWriter writer = XmlWriter.Create(sb, settings);

        writer.WriteStartDocument();
        writer.WriteStartElement("Root");

        foreach (Newsletter nls in nl.GetNewsletterList())
        {
            writer.WriteStartElement("Newsletter");
            writer.WriteElementString("Title", nls.Title);
            writer.WriteElementString("Created", nls.Created.ToString());
            writer.WriteEndElement();
        }

        writer.WriteEndElement();
        writer.WriteEndDocument();
        writer.Flush();

        XmlDocument xmlDocument = new XmlDocument();
        xmlDocument.LoadXml(sb.ToString());
        return xmlDocument;
    }