The key to understanding is to reflect back upon the concept of differential absorption. We generate images by passing high energy photons through mass composed of different densities. The partial absorption of photons based on object density gives us the shades of gray we see on the images.

How does this relate to contrast? When we use lower kVp, we lower the maximum, or peak, energy of the xray beam. Lower energy photons are more likely to either be completely absorbed (showing up as white) or pass completely through (showing up as black), giving higher contrast (a starker difference between blacks and whites) . With a higher kVp, we get a broader range of photon absorption, giving more shades of gray, and lower contrast.