The Car Connection conveniently listed all the fuel economy ratings for each Kona variant and the AWD models see a substantial reduction in mileage, though I'd still go for it.
Kona 2.0 front-wheel drive: 28 mpg city, 32 highway, 30 combined
Kona 1.6T front-wheel drive: 27/33/30 mpg
Kona 2.0 all-wheel drive: 26/29/27 mpg
Kona 1.6T all-wheel drive: 25/30/27 mpg