My phone Battery very hot

My experience says this is purely caused by some third-party app that you may have installed.

I am not aware of the ambient temperature in which you are using the iPhone. However --> Keeping iPhone, iPad, and iPod touch within acceptable ...

Store the device where the temperature is between -20º and 45º C (-4º to 113º F). Don’t leave the device in your car, because temperatures in parked cars can exceed this range.

Use iOS devices where the ambient temperature is between 0º and 35º C (32º to 95º F). Low- or high-temperature conditions might cause the device to change its behavior to regulate its temperature. Using an iOS device in very cold conditions outside of its operating range might temporarily shorten battery life and could cause the device to turn off. Battery life will return to normal when you bring the device back to higher ambient temperatures. Using an iOS device in very hot conditions can permanently shorten battery life.

A phone getting extremely hot can be caused by a number of things. An app gone rogue, as SravanKrA has suggested, is one possibility. Another is that there is some sort of hardware failure, most likely the battery. If the phone is too hot to hold, turn it off, unplug it and make arrangements to have it serviced. Do not use it until it has been looked at as it could be a fire hazard.