This guide have been tested on Singeplayer without upgrades.

Enemy: EASY Time to kill (Pistol - Knee): 5 shots Time to kill (Pistol - Head): 2 shots

NORMAL/HARD Time to kill (Pistol - Knee): 9 shots Time to kill (Pistol - Head): 3 shots

Player: EASY Time to die (Grenade launcher): 3 grenades Time to die (Enemy pistol): 2 clips and 6 shots

NORMAL/HARD Time to die (Grenade launcher): 2 grenades Time to die (Enemy pistol): 1 clip and 2 shots

Conclusion:

• Enemies take more shots to die on difficulties above Easy.
• Enemies are much more accurate and do more damage to the player.
• The player takes more damage on difficulties above Easy.
• I think the only change for Hard is even more accurate AI compared to Normal, though not sure.