I always knew == [pronoun] + [adverb] + [past]
Always is an adverb that indicates all the time; continuously; uninterruptedly.
However, in english, "knew" is the past simple, indicating action that occurred in the past and which does not extend into the present. This overrides the 'continuous and still happening' nature of the adverb "always".
This sentence therefore implies that you knew something, you knew it continuously and at some point (say last Tuesday) you said "I have always known", however that was last week! And the action is no longer relevant, you are now referring to the event in the past and the even has ceased to occur. For example "Last week I told the project manager that I had always known". Of course this is no longer relevant so now you say "back then I always knew".
I've always known == I have always known == [pronoun] + [auxiliary verb] + [adverb] + [perfect]
This is then the perfect tense where something that started in the past and continues in the present. Believe it or not, english is a bit simpler than spanish in that we rarely use preterite or imperfect and only really differentiate between things that have finished (past simple) and things that are still relevant (perfect).