sir in the market three types of ml khanna are available:- 1)ml khanna objective 2)iit mathematics ml khanna 3)new pattern mathematics for iit jee ml khanna

which one should i buy and what is the difference between them?

i recommend iit mathematics by M.L.KHANNA and J.N. sharma 

because the book explains every concept deeply with its quite lithe and lurid initial explanation  + key features like problem sets , which are equiped with all time favourite IIT and AIEEE questions + every problem set has answers with a very attractive way to work out so that you dont face a problem of running to a third person.i think this book will gave you a sufficient initial start u want to start up with. .............................but if u are thinking of practise and u are already through up with all concepts then no doubt u can go for new pattern mathematics for iitjee ml khanna .