what is the problem actually, is there any problem in my connection and should i connect any external power supply to pin number 8 instead of 5V from Arduino for PWM
You should swap arduino pins 10 and 11 so L293-Pin-1 goes to arduino pin 10.
and L293-Pin-2 => arduino d11
Also, connect leds to pins 9,10 and 11 of arduino with 220 ohm resistors so you can see the signal states.
Your biggest mistakes is that L293 PIN-8 IS MOTOR POWER NOT LOGIC POWER !!!
PIN -16 OF L293 IS LOGIC POWER !
DIDN'T YOU EVEN LOOK AT THE DATASHEET ?
l293d.pdf (372 KB)