Change specific cells in Listview (XAML)

How could I change specific cells in a ListView in xaml?  When I use <ListViewItem>aaa</ListViewItem>... it changes the whole row.  Here is my code:

<Grid><ListView Height="156" HorizontalAlignment="Left" Margin="79,58,0,0" 
                  Name="LView1" VerticalAlignment="Top" Width="305"><ListView.View><GridView><GridViewColumn Header="Col1" Width="70"/><GridViewColumn Header="Col2" Width="70" /><GridViewColumn Header="Col3" Width="70" /></GridView></ListView.View><ListViewItem>aaa</ListViewItem><ListViewItem>bbb</ListViewItem><ListViewItem>ccc</ListViewItem></ListView></Grid>