  Optical spectroscopy and density-functional calculations reveal electronic
properties of the nonmagnetickagome metal KV$_3$Sb$_5$. Temperature and
frequency-dependent optical measurements down to 10K and up to 2 eV energy
range confirm bulk nature of the charge-density-wave (CDW) state below 78 K and
gauge the charge gap of $\Delta_{CDW} \approx$ 60 meV at 10 K. We further
detect strong phonon anomalies and the prominent low-energy localization peak
indicative of the unconventional charge transport caused by electron-phonon or
electron-electron interactions. Possible CDW structures of KV$_3$Sb$_5$, the
star and hexagon (inverse star), are strongly reminiscent of $p$-wave states
expected in the Hubbard model on the kagome lattice at the filling level of the
van Hove singularity. The proximity to this regime may have intriguing and
far-reaching implications for the physics of KV$_3$Sb$_5$ and related
materials.
