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In this third paper of the MOVES (Multiwavelength Observations of aneVaporating Exoplanet and its Star) programme, we combine Hubble SpaceTelescope far-ultraviolet observations with XMM-Newton/Swift X-ray observationsto measure the emission of HD 189733 in various FUV lines, and its soft X-rayspectrum. Based on these measurements we characterise the interstellar mediumtoward HD 189733 and derive semi-synthetic XUV spectra of the star, which areused to study the evolution of its high-energy emission at five differentepochs. Two flares from HD 189733 are observed, but we propose that thelong-term variations in its spectral energy distribution have the mostimportant consequences for the environment of HD 189733b. Reduced coronal andwind activity could favour the formation of a dense population of Si$^{2+}$atoms in a bow-shock ahead of the planet, responsible for pre- and in-transitabsorption measured in the first two epochs. In-transit absorption signaturesare detected in the Lyman-$\alpha$ line in the second, third and fifth epochs,which could arise from the extended planetary thermosphere and a tail ofstellar wind protons neutralised via charge-exchange with the planetaryexosphere. We propose that increases in the X-ray irradiation of the planet,and decreases in its EUV irradiation causing lower photoionisation rates ofneutral hydrogen, favour the detection of these signatures by sustaining largerdensities of H$^{0}$ atoms in the upper atmosphere and boostingcharge-exchanges with the stellar wind. Deeper and broader absorptionsignatures in the last epoch suggest that the planet entered a differentevaporation regime, providing clues as to the link between stellar activity andthe structure of the planetary environment.

MOVES III. Simultaneous X-ray and ultraviolet observations unveiling the variable environment of the hot Jupiter HD 189733b