This thesis presents an extensive study of X-ray flux and spectral variability in TeV-detected high-energy-peaked BL Lac objects (HBLs) using archival XMM-Newton observation data. Since the synchrotron peaks of HBLs lie in the UV to soft X-ray band, their X-ray emission provides key insights into the high-energy particle population in relativistic jets near the central supermassive black hole. This work combines three complementary investigations: an intraday flux-variability study of 10 TeV HBLs using 25 observations, a spectral study of 13 TeV HBLs using 54 observations, and a source-specific spectral study of PG 1553+113 based on 30 observations.

Intraday variability was examined in the soft (0.3--2.0 keV), hard (2.0--10 keV), and total (0.3--10 keV) bands. We found that 15 of 25 observations across seven sources showed intraday variability. However, hardness-ratio analysis indicated that most flux changes were not accompanied by significant spectral variations, suggesting largely achromatic variability. Power spectral density analysis of the variable light curves yielded slopes in the range 1.1--2.9, and minimum-variability timescales were used to constrain the magnetic field strength, the size of the emission region, and the Lorentz factor of the radiating electrons. Spectral analysis over 0.6--10 keV showed that most spectra are well described by convex log-parabolic or simple power-law models, consistent with synchrotron-dominated emission. A few observations of PKS 0548-322, Mrk 501, and PG 1553+113 showed signatures of an additional inverse Compton component. Joint fitting of X-ray and Optical Monitor data for PG 1553+113 enabled estimation of its synchrotron peak in the optical/UV regime.