HSV is an alternative numbering system for RGB colors in computer software. Photoshop uses it, too, but V (=value) is named B (=brightness) in Photoshop.
V (=Photoshop's B) is calculated from RGB numbers with the following formula:
V = the biggest of R, G and B divided by 255
V doesn't present actual amount of produced light, because RGB=128,127,128 has the same V (=50% if rounded to integer) as RGB=128,0,0
Computer screens have an adjustment named Brightness. It affects the amount of light produced by the backlight of the screen or it modifies the apparent brightness with some other method. The exact method and how it is presented in numbers is proprietary. Thus it's useless to compare brightness settings between screens.
More: As soon as one modifies the brightness adjustment of their display, they ruin its color calibration (if it was calibrated).
CIELAB color system (=Photoshop's LAB) uses L, which is often said to present perceived lightness. But it's not absolute, it's compared to a standard illuminant. That illuminant isn't absolutely fixed in the CIELAB system, but one used is named D50, which is an attempt to simulate diffuse daylight at certain conditions.
I have read from several sources that Adobe uses D50 as its reference white in Photoshop's LAB color mode. This is one of the sources: http://www.color-image.com/2011/10/the-reference-white-in-adobe-photoshop-lab-mode/
L=100% means the color is seen by the assumed standard watcher as bright as D50. It's not direct sunlight, it's more like what's seen in the sky near the horizon in the middle of the day.
If one gives certain CIELAB color numbers, he assumes certain color and relative lightness. A computer+program+display can produce that color if it's possible to produce in that system and the system is properly calibrated, but the meaning of the numbers LAB is known as soon as one knows the used standard illuminant.
RGB numbers or their equivalent HSV numbers mean a certain color only after one has also specified the used RGB implementation (sRGB, NTSC, Adobe RGB etc...). So, if you have two monitors which are both calibrated to show for ex.sRGB range absolutely right, you can believe both of them show sRGB range in the same way.
Absolute also means that the maximum luminous intensity given by RGB = 255, 255, 255 should be as said in sRGB specs. It says that max bright white should create luminous intensity 80 candelas/square meter.