CURRENT_TIME is a struct timespec.  (How is it updated?  May a read turn out to
actually be compiled into a special instruction that reads a high-accuracy
clock on the CPU?) It's absolute time (unix time, since the start of 1970).

jiffies is lower-resolution (depends on kernel version, architecture, but
roughly milliseconds or tens of) and isn't absolute time.  It could also wrap
around every now and then; if you depend on comparing jiffies values, you may
still do so over a short period of time (a few minutes?) but you should
do it by casting to signed long, subtracting, and comparing to zero, to avoid
problems around wrap-around time.  (Well, no, actually, you should just
use time_before() and time_after(), which already do this for you.)

(Actually, if jiffies is milliseconds and 32 bits, that'd be 40-some days to
wrap around.)

TSC: time stamp counter: http://en.wikipedia.org/wiki/Time_Stamp_Counter

HPET: high precision event timer: http://en.wikipedia.org/wiki/HPET