If what you want is to generate 20Khz frequency using 16F88, then sure you can do it by using PWM feature of this PIC.

For 20Khz at 4Mhz, using 1:1 prescaler, PR2 should be 49.
At 20Mhz, 1:1, PR=249.



mister_e, pls correct me if I am wrong