Serovar vs Serotype?

Essentially, these terms are interchangeable:

A serovar or serotype is a group of microorganisms or viruses based on the cell surface antigens. Serovars allow organisms to be classified at the sub-species level; an issue of particular importance in epidemiology.

Serovars may be established based on virulence factors, lipopolysaccharides in Gram-negative bacteria, presence of an exotoxin (pertussis toxin in Bordetella pertussis, for example), plasmids, phages, or other characteristic which differentiate two members of the same species.

Salmonella, for example, has 18002000 serovars: Salmonella enterica Serovar Typhimurium, S. enterica Serovar Typhi, and S. enterica Serovar Dublin, to name a few.
Vibrio cholerae, which causes cholera, has 139 serotypes, based on cell antigens. Only two of them produce an enterotoxin and are pathogens: 0:1 and 0:139.