Severe, stern, austere, ascetic can all mean given to or characterized by strict discipline and firm restraint.
Severe is applicable to persons and their looks, acts, thoughts, and utterances or to things (as laws, penalties, judgments, and styles) for which persons are responsible. In all these applications it implies rigorous standards of what is just, right, ethical, beautiful, or acceptable and unsparing or exacting adherence to them; it not only excludes every hint of laxity or indulgence but often suggests a preference for what is hard, plain, or meager.
Very often the word suggests harshness or even cruelty. It is then by extension referable also to things for which persons are not responsible but which similarly impose pain or acute discomfort.
Stern, though it often implies severity when applied to persons or their acts or words, stresses inflexibility or inexorability of temper; thus, a severe judge may appear kindly though dispassionately just, but a stern judge reveals no disposition to be mild or lenient; to be made of stern stuff is to have an unyielding will or an extraordinarily resolute character.
In extended use stern is applied to what cannot be escaped or evaded or to what is harsh and forbidding in its appearance or in its external aspects.
Austere is chiefly applied to persons, their habits, their modes of life, the environments they create, or the works of art they produce; in these applications austere implies the absence of appealing qualities (as feeling, warmth, color, animation, and ornament) and therefore positively implies dispassionateness, coldness, reserve, or barrenness.
Sometimes the word tends to add such connotations as restraint, self-denial, economy of means, and stark simplicity and becomes a term of praise rather than of depreciation.
Ascetic implies laborious and exacting spiritual training or discipline, self-denial, abstention from what is pleasurable, and even the seeking of what is painful or disagreeable. The idea of discipline, especially by abstention from what is pleasurable or easy or self-indulgent for the sake of spiritual or intellectual ends may be emphasized.