IF OBJECT_ID('tempdb..#M') is not null 
  DROP TABLE #M
  
CREATE TABLE #M(
                id int, 
                mod1 varchar( 50 ), 
                mod2 varchar( 50 ), 
                mod3 varchar( 50 ), 
                struct xml)

INSERT INTO #M VALUES( 1 , 'M205',   'M206', 'R15', '<Bike ProductID="1" ProductName="Road Bike"/>');
INSERT INTO #M VALUES( 2 , 'Rov-Z1', NULL,   NULL,  '<Bike ProductID="2" ProductName="Mount Bike"/>');
INSERT INTO #M VALUES( 3 , NULL,     NULL,   NULL,  '<Bike ProductID="3" ProductName="Simple Cross-Bike"/>');

SELECT * FROM #M

<Bikes>
  <Bike ProductID="1" ProductName="Road Bike">
    <Model ModelNumber="1" Name="M205" />
    <Model ModelNumber="2" Name="M206" />
    <Model ModelNumber="3" Name="R15" />    
  </Bike>
  <Bike ProductID="2" ProductName="Mount Bike">
    <Model ModelNumber="1" Name="Rov-Z1" />
  </Bike>
  <Bike ProductID="3" ProductName="Simple Cross-Bike" />
</Bikes>

  SELECT struct as '*',
   1  as 'Bike/Model/@ModelNumber',
  mod1 as 'Bike/Model/@Name'
  FROM #M 
  FOR XML PATH(''),
  ROOT('Bikes')

<Bikes>
  <Bike ProductID="1" ProductName="Road Bike" />
  <Bike>
    <Model ModelNumber="1" Name="M205" />
  </Bike>
  <Bike ProductID="2" ProductName="Mount Bike" />
  <Bike>
    <Model ModelNumber="1" Name="Rov-Z1" />
  </Bike>
  <Bike ProductID="3" ProductName="Simple Cross-Bike" />
  <Bike>
    <Model ModelNumber="1" />
  </Bike>
</Bikes>