kVp and mAs are controls you adjust at the console
density and contrast are things on the x-ray that are adjusted by the above as well as other factors like what part you're x-raying
mA and mAs= same. s = seconds so you just multiply them
Contrast = difference between white and black on film. If an x-ray, like a hand x-ray, has a big difference between the white and blacks and has a short scale (aka hardly any shades of grey) on it, it has a high contrast. An abdomen x-ray has many shades of grey and therefore has low contrast and a long scale of contrast.
High contrasthttp://www.shutterstock.com/pic-51884233/stock-photo-x-ray-of-human-finger-side-and-front-view-on-black-background.html
low contrast (first x-ray)
https://online.epocrates.com/u/2911995/Ileus
see the difference?
Kilovoltage is related to contrast. As kVp increases contrast decrease. As kVp increases the energy of the electrons in the x-ray tube increasing both the energy of the x-ray beam and the probability that more electrons will produce x-rays. If the x-ray energy increases the x-rays can penetrate more anatomy and more beams hit the film all around, causing everything to get darker, therefore there is less contrast probably making it a low contrast film. As kVp decreases contrast increases. On the other hand, if the kVp is low, lower energy x-ray beams will be produced and more will be absorbed on its way to the x-ray film.
kVp also affects density but on a much lower scale than mAs (which I will explain in a second). But just keep this in mind.
kVp is the primary factor for contrast
Density - It is important to realize that density can be referring to two things, the density of the anatomy and the density of the film. If contrast is used hand in hand (like your question) than it is referring to density on the film. The other density is how dense: the anatomical number a substance is such as bone. If something on an x-ray is very light, or white, you would say it has low density. Something such as air in the stomach that would appear black on an x-ray would have high density.
In this picture the lungs have a high density and the surrounding tissues, the heart, and the ribs have low density
http://www.flickr.com/photos/aidan_jones/1438403889/
mAs - mAs (or mA) is the prime factor in controlling density. mAs controls how many electrons are produced at the cathode. The more electrons produced at the cathode the more x-rays that will be produced at the anode. The more x-ray beams then more x-rays will hit at each part of the x-ray where it was already hitting. Since mAs has nothing to do with the energy of the beam it won't penetrate any different parts of the body, just make the parts it already is darker.
This is more in a sense that if you don't have enough x-rays at all, despite the kVp, the x-ray will be too light due to not enough x-ray beams hitting the film anywhere.
Overpenetrated (first picture) too dark because you want to see details of the bronchial tree in the lung
http://www.ask.com/wiki/Abdominal_hernia
Underpenetrated
http://www.arrs.org/education/cert/demo2/requiredReading.cfm?product_code=C0801_00XX&edu_module=1
If you do one or the other you are losing anatomy information.
If you have any other questions or need further explainations just let me know!