"It is showing that...." is using the present continuous form of the verb "show" to say that something is currently being shown to you. For example, if I was looking on a computer to buy tickets for an event, I may say to you, "It (meaning the computer screen) is showing me that there are two tickets left." Therefore, I am talking about what the computer is showing me at that particular moment. When you use, "It is showing that..." You are always talking about what is, at that very moment, being shown to you. On the other hand, "It is shown..." is a passive form of writing predominately found in academic papers. In fact, you would rarely, if not, never, hear anyone say "it is shown" in casual speech. It sounds overly formal, and is rarely grammatically correct in speech, unless I was making a sentence along the lines of, "Car A passed Car B at the traffic light while the light was still red, thus it is shown that Car A is at fault for the accident." Do you see what I mean? It sounds like a lawyer, doesn't it? In other words, unless you are a lawyer, or writing an academic paper, you will rarely have the opportunity to hear yourself say, "it is shown."

Which one to use depends on context, particularly what 'it' is referring to. 'It is shown that' is a standard passive academic phrase. 'It is showing that' is less common, and requires that 'it' is a thing that clearly displays something.

Showing is present tense and shown is past tense (currently showing vs. has/ had shown)