Hello shahbaz
Basically Different pon standards are explained by ITU according to some parameters like DATA rate,which can supported by a PON system for 20/40Km and different no. of users.Here i have given links of some good research articles.I would recommend you to go through these papers carefully
http://info.ovum.com/uploads/files/TWDM_PON_white_paper.pdf
http://electronicdesign.com/what-s-difference-between/what-s-difference-between-epon-and-gpon-optical-fiber-networks
http://www.cisco.com/web/strategy/docs/gov/gpon_paper.pdf
http://www.arrisi.com/dig_lib/white_papers/_docs/ARRIS_Comparing_IEEEE_pon_and_FSAN_wp.pdf