Security proofs of QKD protocols were further extended to explicitly accommodate the imperfection in practical devices. Quantum Key Distribution (QKD) provides a way for distribution of secure key in at least two parties which they initially share. And there are many protocols for providing a secure key i.e. BB84 protocol, SARG04 protocol, E91 protocol and many more. In this paper, all the concerned protocols that share a secret key is explained and comparative study of all protocols shown.