TClassA = class
  private
    FInt: Int32;
    FLong: Int64;
  end;

  TClassB = class
  private
    FStr: string;
  end;

  TMyClass = class
  private
    FA: TClassA;
    FB: TClassB;
  end;

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    function ConvertVariantToValue(const aValue: Variant; aRttiType: TRttiType): TValue;
    procedure FillField(aObject: TObject; aTypeInfo: PTypeInfo; aNode: IXMLNode);
    function CreateClass(aNode: IXMLNode; aTypeInfo: PTypeInfo): TValue;
  public
    function Deserialize<T>(aNode: IXMLNode): T;
  end;

{ TForm1 }

procedure TForm1.Button1Click(Sender: TObject);
var
  Obj: TMyClass;
begin
  Obj := Deserialize<TMyClass>(LoadXMLData('<TMyClass><FA><FInt>5</FInt><FLong>8</FLong></FA><FB><FStr>asd</FStr></FB></TMyClass>').DocumentElement);
  // В Obj будет экземпляр нашего класса.
end;

function TForm1.ConvertVariantToValue(const aValue: Variant; aRttiType: TRttiType): TValue;
var
  Temp: Variant;
  Value: TValue;
begin
  TValue.Make(nil, aRttiType.Handle, Value);
  Temp := Value.AsVariant();
  Temp := VarAsType(aValue, TVarData(Temp).VType);
  Exit(TValue.FromVariant(Temp));
end;

function TForm1.CreateClass(aNode: IXMLNode; aTypeInfo: PTypeInfo): TValue;
begin
  Exit(TRttiContext.Create().GetType(aTypeInfo).AsInstance.MetaclassType.Create());
end;

function TForm1.Deserialize<T>(aNode: IXMLNode): T;
var
  Value: TValue;
begin
  Value := CreateClass(aNode, TypeInfo(T));
  FillField(Value.AsObject(), TypeInfo(T), aNode);
  Exit(Value.AsType<T>());
end;

procedure TForm1.FillField(aObject: TObject; aTypeInfo: PTypeInfo; aNode: IXMLNode);
var
  Rt: TRttiInstanceType;
  Rf: TRttiField;
  Value: TValue;
  Value2: TValue;
begin
  Rt := TRttiContext.Create().GetType(aTypeInfo).AsInstance;
  for Rf in Rt.GetDeclaredFields() do
  begin
    case Rf.FieldType.TypeKind of
      TTypeKind.tkInteger,
      TTypeKind.tkInt64,
      TTypeKind.tkUString:
        begin
          Value := ConvertVariantToValue(aNode.ChildNodes.FindNode(Rf.Name).NodeValue, Rf.FieldType);
        end;
      TTypeKind.tkClass:
        begin
          Value := CreateClass(aNode.ChildNodes.FindNode(Rf.Name), Rf.FieldType.Handle);
          FillField(Value.AsObject(), Rf.FieldType.Handle, aNode.ChildNodes.FindNode(Rf.Name));
        end;
    end;
    Rf.SetValue(aObject, Value);
  end;
end;