Peak kilovoltage (kVp) is the maximum voltage applied across an X-ray tube. It determines the kinetic energy of the electrons accelerated in the X-ray tube and the peak energy of the X-ray emission spectrum. The actual voltage across the tube may fluctuate.
kVp controls the resulting photographic property called "radiographic contrast" of an x-ray image (the amount of difference between the black/whites). Each body part contains a certain type of cellular composition which requires an x-ray beam with a certain kVp to penetrate it. The body part is said to have "subject contrast" (that is, different cellular make up: some dense, some not so dense tissues all within a specific body part). For example: bone to muscle to air ratios in the abdomen differ from that of the chest area. So the subject contrast is said to be higher in the chest than in the abdomen. In order to image the body so that the maximum information will result, higher subject contrast areas require a higher kVp so as to result in a low radiographic contrast image and vice versa.
Although the product of tube current and exposure time, measured in milliampere-seconds (mA·s), is the primary controlling factor of radiographic density, kVp also affects the radiographic density indirectly. As the energy (which is proportional to the peak voltage) of the stream of electrons in the x-ray tube increases, the x-ray photons created from those electrons are more likely to penetrate the cells of the body and reach the image receptor (film or plate), resulting in increased radiographic density (compared to lower energy beams that may be absorbed in the body on their way to the image receptor). However, scattered x-rays also contribute to increased radiographic density: the higher the kVp of the beam, the more scatter will be produced. Scatter is unwanted density (that is, density created without bringing any pertinent information to the image receptor). This is why kVp is not primarily used to control density – as the density resulting from increasing kVp passes what is needed to penetrate a part, it will only add useless information to the image.
Increasing mAs causes more photons (radiation) of the particular kVp energy, to be produced. This is helpful when larger parts are imaged, because they require more photons. The more photons you can get to pass through a particular tissue type (whose kVp is interacting at the cellular level) will result in a statistically increased amount of photons reaching the image receptor. The more photons that pass through a part, and reach the image receptor with pertinent information - the more useful the density is created on the resulting image. Conversely, lower mAs creates less photons, which will decrease density, but is helpful when you image smaller parts.