Shimano XTR V-Brake and XT V-Brake look similar, but XTR cost is triple than XT. What is the different?

Answer 1

Weight and level of finish. The XTR series is a few grams lighter and the tolerances are tighter during construction. For 99% of riders, the XT series is more than adequate and actually a bit more durable.

Answer 2

Biggest difference is XT is parts are mostly "Hot Forged" parts and XTR are "Cold Forged" parts.
Cold forging is more expensive, and time consuming process, it aligns the grain of metal to give it more strength, so less metal is used. Giving it lighter weight.
The parts also have more finish work, look better up close.
For most riders XT is plenty strong and more than up to the task. For professionals, elite riders and those with deep pockets that want the very best, XTR is the ticket.