The analyses of X-ray emission from classical novae after outburst have shown that the soft X-ray emission below 1 keV, which is thought to originate from the photosphere of the white dwarf, is inconsistent with the simple black body model of emission. Thus, the ROSAT Position Sensitive Proportional Counter (PSPC) data (0.1-2.4 keV) of the classical novae V1974 Cygni 1992 and some others have been reanalyzed in order to understand the spectral development of novae in the X-ray wavelengths during the outburst stage together with the effect of the enhanced metal abundances in the white Dwarf (WD) atmosphere prior to the outburst. The X-ray spectrum of V1974 Cygni 1992 is fit with a hot white dwarf atmosphere emission model developed for the remnants of classical novae near the Eddington luminosity and a thermal plasma emission model to account for the harder Xray tail in the spectrum of the nova. The post-outburst X-ray spectrum of the remnant white dwarf is examined in the context of evolution on the Hertzsprung-Russell diagram using an O-Ne and a C-O enhanced atmosphere emission model. A constant bolometric luminosity evolution is detected for the first time for a classical nova with increasing effective temperature and decreasing photospheric radius using the O-Ne enhanced model. A peak effective temperature of 51 eV (5.9x10(5) K) is detected at day 511 after outburst. The light curve of the hard X-ray component is found to be independent from the soft one. The maximum of the hard X-ray emission is reached at day similar to 150 after the outburst. The time evolution of the hard X-ray flux and the plasma temperatures decreasing from 10 keV to 1 keV suggest emission from shock heated gas as the origin of the hard X-ray component.