  Detection of X-rays from classical novae, both in outburst and post-outburst,
provides unique and crucial information about the explosion mechanism. Soft
X-rays reveal the hot white dwarf photosphere, whenever hydrogen (H) nuclear
burning is still on and expanding envelope is transparent enough, whereas
harder X-rays give information about the ejecta and/or the accretion flow in
the reborn cataclysmic variable. The duration of the supersoft X-ray emission
phase is related to the turn-off of the classical nova, i.e., of the H-burning
on top of the white dwarf core. A review of X-ray observations is presented,
with a special emphasis on the implications for the duration of post-outburst
steady H-burning and its theoretical explanation. The particular case of
recurrent novae (both the "standard" objects and the recently discovered ones)
is also reviewed, in terms of theoretical feasibility of short recurrence
periods, as well as regarding implications for scenarios of type Ia supernovae.
