What are the differences in maps between difficulty levels?

All maps have different variation for each time you play it, and the difficulty will effect what appears in the map.

The things that change relating to the difficulty is things like ATM's, safe's, large safe, camera's, objectives, additional loot, etc.

For example on Four Store on difficulty normal - overkill there is normally atleast 1 ATM on the level, but in Deathwish the possibility of getting an ATM is extremely low if not impossible.

The same rule applies with security features. for example when playing the Ukrainian job on noraml - hard you will have normal safes where the tiara is store and when you go to play on overkil there will be Titan safes which cannot be blown up. and on Deathwish there will always be titan safes and potentially titan cameras which cannot be destroyed.