Can a bike have both Schrader and Presta tires? I have Kona Dew 2018 and the front tire has a Presta valve while the back tire has a Schrader valve. Is it common to see bikes have different valves for each tire?
Short answer, it's not common at all and probably wasn't like that when the bike was new. 

The valve that you need is determined by the rim rather than the bike. If the rim is drilled out with a big enough hole for a schrader valve, then that's what you should use. If a schrader valve doesn't fit through the valve hole, then you should use a tube with presta valve. If one of the rims on the bike has been replaced (or drilled out), then they may no longer match and you would need one of each.

The more likely scenario is that at some point one of the tubes was replaced and instead of getting the correct schrader valve tube, a presta was used instead. This is easy to check as the presta valve will not fit snugly in the valve hole. If this is the case, it will work well enough in the short term, but I'd still recommend replacing it with the correct tube with a Schrader valve. Over time, the rubber around the thinner presta valve will rub against the edges of the larger diameter valve hole in the rim and eventually wear out.