What is the difference between builtin voltage and cutin voltage in pn diode?
I've not heard the term 'cutin', or perhaps 'cut-in', voltage before. This is more commonly called the 'threshhold' voltage.

It will refer to the forward voltage that is commonly held to be necessary to get a forward biased diode to conduct. For silicon, this is often given as 0.7v, with shottky and germanium being around 0.3v. LEDs have higher voltages from 1.6v for red to 3v for blue.

In fact, diodes conduct at all potentials, including reverse. It's just that the conduction tends to be insignificant in the context of typical electronic circuits needing (say) a \$10\mu A\$ to 10mA range, for which the voltage will be close to the above mentioned voltages. For the accuracy that the temperature coefficient permits, most engineers say that diodes conduct at the threshhold voltage, and understand that they are dealing in a crude approximation.

The 'built-in' voltage refers to the difference in energy of the doped semiconductors that form the diode junction. See wikipedia's entry for p-n junction. This is constant, is a function of the doping densities of the semiconductors, and is of the same sort of order as the threshhold voltage, which gives rise to the confusion.