How can I pass a XMLNode parameter to a webservice from my ajax client

How can I pass a XMLNode parameter to a webservice.
My Webmethod is like the following:

 [WebMethod]
        [ScriptMethod(ResponseFormat = ResponseFormat.Xml)]
        public string HellowWorldXml(System.Xml.XmlNode xml)
        {

            return xml.InnerXml;
        }

I am trying to call my method from my javascript like the following:

var strXml =  "<Method ID='1' Cmd='Update'><Field Name='ID'>1</Field><Field Name='Title'>testupdate</Field></Method>"
myProxy.HellowWorldXml(strXml, onSucceeded, onFailed);

Is there any other way I should call a webmethod when the input parametr is XMLNode
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