zen being influenced heavily by chinese and then japanese mind sets which themselves were largely influenced by taoism, confucianism and shintoism respectively is largely naturalistic and practical in nature. it relies heavily on concepts such as oneness with nature or the universe, actualizing "the way" (in essence, this is rarely said outright) which is largely similar to taoism. meditation is usually just letting the mind be, which is supposed to lead one to nirvana which is an unconditioned, natural state.
theravada is more influenced by indian mindsets from it's time of inception which are more meticulous and specific in practice and teaching. they use lists and numerical progressions and clearly laid out and defined step by step meditation procedures and so on. as opposed to silent illumiation and shikantaza are both methods in which progressive mind state teaching is not given.
it is very similar to vipassana in it's focus on mindfulness and non jhana sitting meditation, however unlike vipassana it does not use much contemplation. everything is thoughtless and without much direction. by thoughtless i mean one is usually not taught to contemplate not self, death and what not. one is just taught to sit. either literally just sit with no guidance or just sit with the breath. by direction i mean there is no listing of progressive states one can achieve such as the theravada stages of purification or the jhana progression. the only time one is taught to work with thoughts as contemplation is for koan study.
they teach from a few mahayana sutras such as the lankavatara sutra, heart sutra and lotus sutra among others while theravada uses the entire pali canon. the mahayana sutras are so massive and varied that one school rarely proclaims and uses all sutras within it as this would cause a lot of confusion and conflict. for example the pure land school sutras talk about how one can go to amitabha's pure land after death and reach enlightenment there by having faith in him, whereas other mahayana sutras talk more about self effort and direct practice to get oneself to nirvana in this life as opposed to the next or some kind of pure land, the lotus sutra proclaims itself the ultimate and highest sutra and so on. if you tried to practice all the mahayana sutras teachings you would end up in hundreds of knots. the pali canon is more or less internally consistent.
i do not know of a single practice found in zen, aside from koan, that is not found in theravada (and some would argue that there are things similar to koan in theravada as well). however there are many practices found in theravada that are not found in zen.