Verbs мыться (to wash the whole body) and умываться (to wash the face) are different and both imperfective.

imperfective - perfective:
мыться - помыться
умываться - умыться

Here is the way of verb formation:

Root + ть(ся)= imperfective
Prefix + root + ть(ся) = perfective
Prefix + root + suffix ва + ть(ся) = again imperfective

There are many prefixes with specific meanings and to make imperfective couple to a specific perfective verb, suffix ва is used.