Generally, psychosis is a symptom whereas schizophrenia is a disorder. Schizophrenia has symptoms other than just psychosis. Psychosis can be a part of various different disorders, for example: bipolar, schizoaffective disorder, post-natal psychosis, and psychotic depression. Sometimes psychiatrists might give a patient a diagnosis of psychosis but generally, at least from what I've seen, that's a working diagnosis not a concrete one -> psychosis doesn't appear as an individual disorder in the DSM but rather as a component of other ones.