Wides and no-balls are known as bowling extras and are counted against the bowler (think of the bowler being at fault for them), and therefore an over that includes any bowling extras cannot be a maiden over.
Byes and leg-byes are, by contrast, known as fielding extras (think of the fielders being at fault - the wicketkeeper failing to take the ball, for instance) and do not count against the bowler, so a maiden over may include byes and leg-byes.
A maiden over, in short, is an over containing neither any runs off the bat nor any bowling extras (wides and no-balls).
Tom Smith's Umpiring and Scoring (one of the foremost, possibly the foremost, reference books on umpiring and scoring in cricket) has this to say:
A maiden over is one where no runs are conceded by the bowler, i.e. the striker has no scored any runs and there have been no bowling extras (No balls or Wides). It may include fielding extras (Byes, Leg byes or penalty runs).