Are you speaking of CustomListView or ListView.
For CustomListView you got the answer from DonManfred.
In ListView for single line use:
ListView1.SingleLineLayout.Label.Typeface = Typeface.DEFAULT_BOLD
The same principle is valid for the other layouts.
But, be aware that these settings are valid for all lines with the same layout (SingleLineLAyout,TwoLinesLayout or TwoLinesAndBitmapLayout).