Ajax at Client :

  var request = new XMLHttpRequest();
  var xmlstr = "<persons><person><name>user1</name><age>12</age></person><person><name>user2</name><age>11</age></person></persons>";
   request.onreadystatechange=<your callback>;
    request.open("Post","AjaxProcessor.aspx",true);        
    request.setRequestHeader("Content-Type","Text/XML");
    request.send(xmlstr);

Server AjaxProcessor.aspx:

using System.Xml;

       try
        {
            XmlDocument doc = new XmlDocument();
            doc.Load(Request.InputStream);       
        }
        catch (Exception ex )
        {            
            throw;
        }