Stars earlier than about F5 (Teff ~ 8000 K) donŐt have convective envelopes and donŐt have any X-ray emissionÉ
Except that O and early B stars do have X-ray emission - they are strong sources of soft X-rays.  And they have strong stellar winds.
Wind broadened and blueshifted UV absorption lines of an O and a B star.
HST image of h Car; an extreme example of a hot star wind.