What's the difference in meaning?

"It's known that he is a thief." is a correct sentence, meaning he is recognized as a thief.

"It has been known that he is a thief." is not used in English. You'd have to say instead something like: "It is known that he has been a thief." (Meaning: the fact that he once was a thief is recognized.)

tenjingIt's known that he is a thief.

Now. Everybody knows he is a thief.

tenjingIt has been known that he is a thief.

For some time, starting in the past. Everybody has known (since then) that he is a thief.

CJ

The second one is pretty unnatural. I can't think of any reason why one would want to say it.

The first one is normal.

(Edit: I had not seen CalifJim's or Anon's reply when I wrote this.)

What about this one,
It has been acknowledged that underfunding is part of thee problem. Vs. It is acknowledged that underfunding is part of the problem. Is it the same as a above sentence?

These are used in reference to different times of the action of acknowledging, for example:

As we begin this meeting, it is acknowledged (by all here) that underfunding is part of the problem. This meeting therefore has been called to address this part of the problem (underfunding).

It has been acknowledged (previously, by the governor - who can give us no assurance of help with

The reason that "It has been acknowledged that he is a thief." is not used in English and "It has been acknowledged that underfunding is part of the problem." can be used, is that usage-wise, the word "acknowledged" is not used in English to refer to a "thief."