My take on this is that nightvision is being able to take in more light so that less is needed. Darkvision, on the other hand, is an ability to see without a traditional light source, because of magic. Maybe it is a magical-vision, sort of like infravision, only seeing the natural magical emanations that everything (except a very few things) gives off. Either that, or it is sort of Hyper spectral vision that runs the gamut of the spectrum, merging it all to form a combined image - they are not able to switch between the types (infra, ultra, gamma, etc..), it is all one image to them.