Face it. Yamaha did practically NO hardening to the XV valve train. Sometimes I think I see a worn discoloration on the valve adjusters. Sometimes something pops off the cam lobes. But there is no sign of any hardened surface - except the rocker shaft. They are as hard as a pawnbroker's heart. You can clamp down as tight as you can go with a vice grip or similar tool, and make no slightest mark on the surface. But they wear out, too. Hard materials are vulnerable to twisting or turning forces, like the rocking of a tappet.
Valves are no different. It is an automotive standard to harden the tips and faces of valves. These are exactly the parts that show the most wear. Divots on top of the stem; burned faces that clean to ridges that won't clean to a nice forty five. The after-market has hardly acknowledged the XV line, despite there being tens of millions of them. Two of my vendors will modify 500 single valves to fit. Their economy lines run a hundred fifty per valve on up. The cast iron guides seem to last forever. I've put four new sets in guides that remained in spec for half a million miles.
Yamaha still sells these valves. They can't take the heat and they meet the seats eccentrically. Soon they leak, but the bike still runs - poorly. I have found a source of Japanese valves made with tooling that Yamaha had "retired." The stems, tips, and faces are hardened. In appearance, they are identical except they lack the model specifier cast into the face. They are cheaper and more durable. They are not exotic metals or hollow. They are like the valves in a typical car. They are like stock except they require adjustment less often, and don't require being re-ground until your rings wear out.
You may notice a bit of blue smoke upon deceleration when the engine is hot. Or on any high-mileage motor. The stock valve guide seals are no special rubber and wear out quickly. The stock cast iron guides are just about the only part of the valve-train that lasts the way they should, but they require some clearance, and some oil is bound to find its way into the combustion chamber. Viton is a synthetic rubber with many of the characteristics of Teflon - without the work hardening. Resistance is near zero, they are oil tight, and they'll outlast the rest of the top-end.