Está lloviendo. This means: it’s raining. The use of “está” implies that the verb following it should be continuous as “está” refers to an action that is taking place right now. “Llueve” is the simple present form of the verb llover, so it makes no sense in the context of the sentence. It cannot be used with “está”. “Llover” is also wrong as it is the verb in the infinitive. And “lluvia” is not a verb but a noun. It means rain.
Esta is a word describing an action that is happening at that moment in time, and the -iendo ending in Spanish is the equivalent of the -ing ending in English. To say that it's snowing, you would use lloviendo.
