I want to cool my Asus GTX 670 by using side intake fan but my case don't have side intake panel. Is there other way to cool my GPU?

got bottom intake? or top exhaust?
whats ur casing btw

Both got. My casing is Xigmatek Asgard 382.

then it should be sufficient add F12 fans as intake & exhaust

standard or PWM?

Standard for case fan is enough, unless you want silent and has PWM port ready.
Else you can have 5V mod for silent operation but air flow is very minimal.

So Standard isn't silent? What is PWM port for?

Standard can be silent, depending on what kinda fans ur using that is... you could opt for the Artic F12 or the Bitfenix Spectre Pros for good noise/performance ratios, just get the 3-pin version will do if its for intake and exhaust

PWM means Pulse-width modulation and it enables speed control based on the temperature sensor thats' built in to the motherboard.. its a 4-pin header for these fans but most mobos will only have one or two of the 4-pin headers while the rest will be 3-pins