1. Nuclear Weapons
Pakistan has over 130 nuclear warheads capable of targeting every inch of India, there isn't a single place that is safe. These nuclear warheads can be launched by air or land, and make no mistake, India cannot stop them. India's nuclear defences are primitive, in small supply as well as untested. Not to mention if they actually worked India would have invaded Pakistan by now. If you want to know how much damage a single nuclear warhead can do:
https://en.m.wikipedia.org/wiki/Atomic_bombings_of_Hiroshima_and_Nagasaki
Keep in mind, those bombs were manufactured way back in WW2, imagine how much more potent they would be now, and that instead of 2 warheads being used, it would be well over 100. More than enough to destroy India's largest cities.
If these typical nukes aren't scary enough, Pakistan also has tactical nukes that they can and will deploy on the battlefield, giving them the potential to wipe out entire divisions. Not only does it act as a significant force multiplier for Pakistan, but it also shows that Pakistan is very willing to take some dangerous decisions, this alone can be enough of a deterrent.
It's also important to note that unlike India, Pakistan doesn't have a no first use policy, and that Pakistan also has a second strike capability so forget trying to nuke the place first.