When a man with a bass or baritone imitates a female's voice, this is falsetto. The sound he makes is higher-pitched than his normal voice, and it does have a head voice quality to it, but it is not head voice. The lightest form of vocal production that can be produced is falsetto. When singing falsetto, the voice may jump or break and will have an airy sound, because your vocal cords aren't totally closed.
When you make or hear a falsetto, it has an airy tone because of the way it is produced by your vocal cords. The airiness occurs when the tiny vocal folds come close enough to each other to make the edges vibrate as the air flows between the folds, but the folds do not make contact with each other. The airy sound happens when air escapes through the space between the cords.
Head voice means that the tone starts to resonate more in the nasal and sinus cavities on higher notes. When using head voice, the vocal cords stay in contact with one another, which is the biggest difference between falsetto and head voice. The head voice is clean and clear and doesn't have the excessive airy sound that you get in falsetto, and that's because no excess air escapes.
Head voice is a natural high pitch, and the upper range of the voice is utilized. It flows completely and evenly. Sometimes head voice is referred to as head register, because the person singing feels the actual vibrations of the notes in his head. The vocal cords are closed when singing head voice, which makes the tone pure. Head voice results in a smooth harmony and connected sounds, which falsetto doesn't do.