- a killed person. I saw a killed person in the street.
- a person killed. I saw a person killed in the street.
Is there any difference in meaning between two these phrases, and are they both acceptable and interchangeably used in context?
The two usages are not interchangeable, but both are acceptable—provided we supply the singular, uncountable noun street with a determiner!
The difference between the two sentences is a very simple one. In your first sentence, the past participle killed is in the prenominal position (before the noun). This causes it to be interpreted as an adjectival form that modifies the noun person:
I saw a killed person in the street.
This tells us that you saw a person in the street who had been killed.
In your second sentence, the past participle is in the postnominal position (after the noun), and this causes killed to be interpreted as a passive verb form with the noun as its patient:
I saw a person [being] killed in the street.
This tells us that you saw the killing as it was taking place.
In English, past participles frequently serve as adjectives. However, the past participle of kill is seldom used in this way. If we substitute a verb whose participle is more commonly used in this way, the sentences are much more idiomatic:
I saw a frightened person in the street.
I saw a person frightened in the street.
I saw a tattooed person in the street.
I saw a person tattooed in the street.
The first one clearly suggests that the person you saw was the one that was killed and makes good sense for your listeners.
The second sentence, on the other hand, does not make sense the way you have written it.
The verb pattern is "to see somebody DOING something". That inquires the sentence to be:
I saw a person killing in the street. ---> In this case, the person, who you saw, was the one doing the killing.
Another verb pattern is "to see somebody DO something"
I saw a person kill in the street. ---> In this case, you've been watching this person killing in the street from the beginning until the end.
You can either put an infinitive without "to" or a gerund, but a past simple form of the verb has no use in this sentence whatsoever.