Classical Cryptographic systems security relies on the complexity of mathematical problems and limited computational power available at the attacker's end. But with the advent of quantum computers that are assumed to have unlimited computational capability, classical cryptography is now proved to be no more unconditionally secure. It has been proved that the security of a cryptographic system depends on the key used for encryption, not the algorithm. Quantum Key Distribution (QKD) is a key sharing mechanism between two parties whose security relies on the very laws of nature (quantum mechanics), not on the computational capability of the eavesdropper. In QKD, if an eavesdropper tries to gain key information, then he will be detected immediately, and the key sharing procedure is aborted. In this paper, we will survey some of the most prominent QKD protocols proposed till now, and then we will perform a comparative analysis on those protocols depending on various factors.