HSV, for Hue, Saturation and Value, is a color space that defines color by a non-linear approach. The Hue component represents the color in its truest form, essentially in a color wheel. If you were to animate the Hue value alone, you would see something akin to an animated rainbow, with the Saturation and Value consistent, something that is impossible to do with the RGB color space. Saturation refers to how much of the color in the Hue component is displayed. The value component refers to the final brightness of the color itself. This value is presented as a grayscale in typical cases.
HSL, for Hue, Saturation and Lightness, is functionally the same as HSV.