[<DataContract>]
type Responce = {
    [<DataMember>] mutable x : string; 
    [<DataMember>] mutable y : string; }

[<ServiceContract>]
type service() =
      [<OperationContract; WebGet (UriTemplate="/xml/{a}/{b}")>]
        member x.getXml (a, b) = {x = a; y = b}

namespace core.wcf {
    [DataContract]
    public class Responce {
        [DataMember]public string x;
        [DataMember] public string y; 
    }

    [ServiceContract]
    public class service { 
    [OperationContract , WebGet(UriTemplate="/xml/{a}/{b}")]
        public Responce getXml(string a, string b) {
            return new Responce { x = a, y = b};
        }
    }
}

<string xmlns="http://schemas.microsoft.com/2003/10/Serialization/">

[<ServiceContract>]
type service() =
    
    let xn s = XName.op_Implicit(s)
    let xe (n, v:string) = new XElement(xn n, v)
    
    [<OperationContract; WebGet(UriTemplate="/xml/{a}/{b}")>]
    member x.getXml(a, b) = new XElement(xn "Responce", xe("x", a), xe("y", b))

namespace core.wcf {
    [ServiceContract]
    public class service{

        private XElement xe(string n, string v) { return new XElement(n, v); }

        [OperationContract, WebGet (UriTemplate="/xml/{a}/{b}")]
        public XElement getXml (string a, string b) {
            return new XElement("Responce", this.xe ("x", a), this.xe("y", b));                 
        }
    }
}

- <Responce>
      <x> 2 </x> 
      <y> 3 </y> 
  </Responce>

namespace core.wcf {
    [ServiceContract] 
    public class service{

        public class data {
            public string x;
            public string y;
        }

        private XElement xe(string n, string v) {
            return new XElement(n, v);    
        }

        [OperationContract, WebGet(UriTemplate="/xml/{a}/{b}")]
        public string getXml (string a, string b) {
            data d = new data { x = a, y = b };
            DataContractSerializer dcs = new DataContractSerializer(typeof(data));
            using (Stream ms = new MemoryStream()) {
                dcs.WriteObject(ms, d);
                ms.Seek( 0 , SeekOrigin.Begin);
                XmlReader xr = XmlReader.Create(ms);
                XNode xn = XElement.ReadFrom(xr);
                return xn.ToString();
            }            
       }
    }
}

namespace core.wcf {
      
    public class responce {
                public string x;
                public string y;
            }      

    [ServiceContract] 
    public class service{

        [OperationContract, WebGet(UriTemplate="/xml/{a}/{b}")]
        public XElement getXml (string a, string b) {
            responce d = new responce { x = a, y = b };
            DataContractSerializer dcs = new DataContractSerializer(typeof(responce));
            using (Stream ms = new MemoryStream()) {
                dcs.WriteObject(ms, d);
                ms.Seek( 0 , SeekOrigin.Begin);
                XmlReader xr = XmlReader.Create(ms);
                xr.MoveToContent();
                return XElement.ReadFrom(xr) as XElement;             
            }            
        }
    }
}

module xmlns 

open System
open System.Xml
open System.Xml.Linq

let rec remove (x:XElement) =
    let xn s = XName.op_Implicit s 
    match x.HasElements with
    | false -> new XElement(xn (x.Name.LocalName), x.Value)
    | _ -> new XElement(xn (x.Name.LocalName), x.Elements() |> Seq.map remove)

...
return xmlns.remove(XElement.ReadFrom(xr) as XElement);

[WebGet(ResponseFormat = WebMessageFormat.Xml)]