microstepping and pwm
Which is better method to run a stepper motor with maximum possible speed..?


Microstepping


  which can be used to run the motor at almost zero speed ..?


Microstepping


  can we be able to rotate the motor at zero or less than Zero speed with PWM method..?


What is "less than" zero speed? Backwards? Turning through another dimension?  PWM cannot drive a stepper motor. That's like asking if you can download a cake through your suitcase. The concepts just don't match up.  Yes, there are ways of implementing microstepping using closely coupled phase controlled PWM signals, but the end result is microstepping.

A stepper motor requires coils to be activated in the right order and at the right time (and in some cases with the right polarity). PWM just generates a square wave with a set frequency and duty cycle. 

To be clear, a single step of a stepper motor involves cycling through all the correct sequence of energising coils (often a sequence of 4 energisings). Microstepping is merely the act of stopping at different points within that single cycle to give a higher resolution that whole steps (usually 4 times the resolution). It is nothing magical or special.