The best way to compare the Subaru XV Crosstrek to the Subaru Impreza is to compare it to the 2.0i Sport models. So that's what we'll do. A 2015 Subaru XV Crosstrek 2.0i Limited is on the left, and a 2015 Subaru Impreza 2.0i Premium is on the right. This will only cover Exterior differences as you will not notice any difference in the interior or engine setup (the dimensions and setup the same, only options are different).
Exterior profiles of the vehicles show the same body lines. The only exterior difference would be the ride height. The 2015 Subaru XV Crosstrek has 8.7" of ground clearance, same as the Forester and Outback. The 2015 Subaru Impreza has 6.2" of ground clearance.
The front bumper of the 2015 XV Crosstrek is closer to the 2014 Impreza than the 2015. The 2014 Impreza has a similar bumper but with black housing around the fog light instead of the body color paint the XV Crosstrek has. The 2015 Impreza has an updated fog light housing and lower front grille while the 2015 XV Crosstrek remains the same as last year's model.
You'll notice an inverted rear valance on these models. The XV Crosstrek has the black rugged molding that runs along the front and sides ends surrounding the rear bumper corner reflectors while the Impreza has the black molding in the middle between the two reflectors.
The rims on the XV Crosstrek and Impreza are noticeably different. While both 17" designs, the XV Crosstrek has a little bit taller tires to add to the ride height of the XV Crosstrek.
 

Other things worth noting, the XV Crosstrek acheives 34mpg highway while the Impreza reaches 37mpg highway. And while the 2015 model year brought an exterior refresh to the Impreza, the XV Crosstrek exterior remains the same.

The biggest difference between the two can be felt on a test drive. The Impreza rides close to the road like any car would. The XV Crosstrek rides like a car, but with its heighten suspension you have a higher drive height, load height, and step in height. The cars may appear very similar, but driving both will show you the difference.