In my opinion, silence is more "quiet" than quietness. Quiet and "still" (and calm, peaceful, etc) usually go together well. Silent also seems more eerie. In a horror movie, things aren't quiet, they're silent. Hush as a noun is used almost exclusively in literature. And then, yes, I would say a hush suggests that there was noise and now there isn't. Less commonly, hush is used before noise, and then it would be foreboding. Otherwise it's mainly used as a slightly rude command to stop talking.

I studied these three words in my Oxford dictionary and I got the following conclusion of my understanding: Hush is used when there's no noise made by humans. Quietness emphasizes the peaceful atmosphere of the environment, including humans and other creatures. Silence indicates that there's no sound from the humans around, either big noise or soft words.

Just answering on my proper interpretation here. I would assume silence and quietness to be the same as in both meaning absence of sound. Quietness could be used to explicitely describe the presence of calm. In practical use of the words it seems to me that one would rather use the word 'silence' to describe the absence of speech or sounds of live beings, and one would use quietness more to describe the absence of other noises. Keep in mind this is a subjective answer. It's a very safe bet to completely use these two words interchangeably though, so no worries. I looked up 'hush' in the dictionary as it surprised me when you said 'noun'. Apparantly it does mean a silence after noise. But surely, i've never ever heard this word being actually used as a noun. It's a verb almost always used in the imperative form to quiet or to calm down someone, informal in use. Regards, Sertan.