  The Kagome superconductors AV3Sb5 (A=K, Rb, Cs) have received enormous
attention due to their nontrivial topological electronic structure, anomalous
physical properties and superconductivity. Unconventional charge density wave
(CDW) has been detected in AV3Sb5. High-precision electronic structure
determination is essential to understand its origin. Here we unveil electronic
nature of the CDW phase in our high-resolution angle-resolved photoemission
measurements on KV3Sb5. We have observed CDW-induced Fermi surface
reconstruction and the associated band folding. The CDW-induced band splitting
and the associated gap opening have been revealed at the boundary of the
pristine and reconstructed Brillouin zones. The Fermi surface- and
momentum-dependent CDW gap is measured and the strongly anisotropic CDW gap is
observed for all the V-derived Fermi surface. In particular, we have observed
signatures of the electron-phonon coupling in KV3Sb5. These results provide key
insights in understanding the nature of the CDW state and its interplay with
superconductivity in AV3Sb5 superconductors.
