You aren't implementing the ListView data binding correctly. ListView is designed to bind to an object that implements IEnumerable. The items in your combo box must be something that is enumerable like collections, arrays, etc. The ListView needs to bind to the SelectedItem property of the combobox. So something like:
<ListView ItemsSource="{Binding ElementName=cmbID, Path=SelectedItem}">
When a user makes a choice in the combobox the SelectedItem property changes which the ListView will detect via the binding. The ListView will then set it's DataContext to the
selected item. This is why the items in your combobox must all implement IEnumerable. When the ListView sets it's DataContext to the selected item, it will iterate over that item to create an entry for each entry in the selected item. The key here is that your bindings for the columns are
relative to a single item. So they should just be {Binding ID} and {Binding RollNum} etc.