SVT, or supra-ventricular tachycardia, is merely the upper chambers of the heart beating more rapidly than the ventricles below them. It is a class of 'arrhythmia' because it is not normal NSR (normal sinus rhythm). Technically, atrial fibrillation and flutter ARE SVT because the beats happen rapidly, more rapidly than necessary for the body's demands. In windy's case, her ventricles operate in concert, or sympathetically, meaning they speed up as well (RVR, or rapid ventricular response). This was not apparent in my own atrial fibrillation (AF). My heart rate below the coronary sinus, meaning the two large ventricles, was normal. Only my left atrium buzzed along with its irregular rhythm.

PVCs are premature ventricular contractions, also an arrythmia, because the intervals between the R complexes varies, something like in atrial fibrillation where there are no discernible P waves and the intervals of the QRS complexes are irregular in length. In this case, the heart causes the ventricles to beat too often, and not in a regular rhythm....meaning extra beats. Those extra beats mean the patient feels thumping and bumping in the chest, just as most patients with PACs feel (premature atrial contractions).
https://litfl.com/supraventricular-tachycardia-svt-ecg-library/