Contrast is defined as the differences in radiographic densities within an image.
Low kVp causes high contrast (higher differences in "blackness" between adjacent structures)
As you can see, the differences between the adjacent densities are fairly great, especially once you compare it to a low contrast example below.
The above example shows very little difference between the adjacent densities. This is an example of low contrast, or long scale contrast. Higher kVp will cause radiographs to appear more "grey".
As we've learned, kVp controls the energy of the x-ray beam. The higher the kVp, the more penetrating ability it has. So how would this look on an image? If a higher percentage of existing x-rays are making it through the patient and onto the image receptor, the image would appear more grey. From the above images, you can see that the image on the left is more black and white, which means it has higher contrast, caused by less kVp.
Every exam will have an optimal kVp range. Too high of kVp, and you have low contrast (gray). Too little kVp, and the contrast will be too high (black and white).