Kylo Ren is a very strong force user. Luke says something to the effect to Rey:
I wasn't frightened enough when I came across such power the first time
We also see him perform other 'tricks' that have never been seen in Star Wars before, like stopping a blaster bolt in mid air for more than a minute.
However I don't think we've seen what the powers of Snoke are, so cannot judge whether he or Kylo Ren are more capable.
The key thing we see is that Kylo Ren disguises his feelings about what he is about to do. You can see Snoke (apparently) reading Kylo's mind - repeating out loud that he is turning the lightsabre and is about to kill his target. Kylo does this acting out turning the lightsabre in his hands as he stands in front of Rey, while turning the sabre to the side of Snoke with the Force.
He is gambling that Snoke's attention is on what he is seeing in front of him. He is gambling that Snoke will assume the feelings he is reading relate to Rey's imminent death, and overlooks the possibility that his apprentice is planning to kill him.
So its not that Kylo is so strong with the Force that he can disguise his thoughts completely. He uses his powers and misdirection to achieve his aims.