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.
Why not? The bike will roll anyway. You would need two pump heads or an adapter to pump both tires, but other than that, it will work just fine.
There's no mechanical reason why you can't have one of each. The disadvantages are purely practical.
You need a pump that can deal with both valve types, or two pumps, or to mess about with adaptors.
You need two kinds of spare tubes at home.
Most people who cycle longer distances bring a spare inner tube with them because it's much faster to replace the tube than to find and repair a puncture at the roadside (a second puncture on the same ride would need to be repaired). You could just bring a spare presta tube which will certainly get you home if you replace your Schrader tube with it.
Whether any of this is a problem for you depends on your needs and preferences.
Presta valve stems are narrower than Schrader, so you could just replace the Schrader tube with presta and then you'll only have one kind of tube. You could use an adaptor to avoid concerns about the narrow stem moving around in the wide hole.
Check the rim of your presta wheel: it might just be that somebody put a presta tube in a Schrader rim. In that case, you can replace your presta tube with the Schrader tube the rim was intended for.
If both your tubes match the rim they're installed in, you could replace the front wheel (cheaper than the back) with one that takes the same kind of valve as the back wheel. Depending on what kind of bike you have, new wheels might not be all that expensive, though I'm not sure if I'd replace one just to get the convenience of matching valves.
My hybrid currently does, and it's not a problem.
I'm switching over to presta tubes so that the same spares fit that and my tourer. That way I don't have to stock so many types of tube, especially in my commuting pannier.
The presta should be used with a spacer of some sort. I prefer a rubber grommet as the plastic parts sometimes sold for the purpose have caused me problems in the past (hard edges abrading the tube).