How any one achieve sinusoidal back emf in PMSM and Trapezoidal back EMF in BLDC ? What is the winding differences?
The magnets and pole faces can be shaped and positioned to achieve a more 'trapezoid' back-emf. Different winding patterns may also have an effect. 

However I suspect that matching back-emf to the drive waveform is often not done, because real BLDC back-emf waveforms are all over the place. Here are some scope traces showing the phase-to-phase waveforms of 3 motors that I tested (vertical pulses are PWM drive, back-emf is the middle waveform that occurs when the phases are not driven):-

   





These are all small 'in-runner' BLDC motors rated for 100-300 Watts, designed to power RC model aircraft. The first two motors have slotted iron stators. One produces close to trapezoid back-emf, the other nowhere near it. 

The last trace is from a coreless ironless motor, which explains its almost perfect sine wave back-emf. Despite having a 'suboptimal' back-emf waveform, this motor (which only weighs 28 grams) produces 90W at 60,000rpm with 83% efficiency.