I John 1:4

KJV - And these things write we unto you that your joy may be full.  (write we unto you - Byz / your - A, Byz )

NASV - These things we write, so that our joy may be made complete.  (we write -  Aleph, B / our - Aleph, B)

I John 1:7

KJV - But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin. (Byz)

NASV - but if we walk in the Light as He Himself is in the Light, we have fellowship with one another, and the blood of Jesus His Son cleanses us from all sin. (non-Byzantine manuscripts)

I John 2:7

KJV - Brethren, I write no new commandment unto you, but an old commandment which ye had from the beginning. The old commandment is the word which ye have heard from the beginning. (Byz)

NASV - Beloved, I am not writing a new commandment to you, but an old commandment which you have had from the beginning; the old commandment is the word which you have heard. (Aleph, B - probable)

I John 2:20

KJV - But ye have an unction from the Holy One, and ye know all things.  (A, Byz)

NASV - But you have an anointing from the Holy One, and you all know.  (Aleph, B)

I John 2:23

KJV - Whosoever denieth the Son, the same hath not the Father: (but) he that acknowledgeth the Son hath the Father also. (See note.)

NASV - Whoever denies the Son does not have the Father; the one who confesses the Son has the Father also. (See note.)

Note: Normally, KJV translators italicized to clarify the meaning while indicating the words were not in the source text.  In this instance, the words ARE in the source (TR, or Textus Receptus).  However, other sources they considered very trustworthy (BYZ manuscripts) did NOT have the words.  They italicized them to indicate uncertainty about their originality.  NASV translators followed reliable manuscripts (such as Aleph, B, etc.) which were not available to the KJV translators.  Those sources agree with the TR and DO include the words.

I John 3:1

KJV - Behold, what manner of love the Father hath bestowed upon us, that we should be called the sons of God: therefore the world knoweth us not, because it knew him not.  (Byz)

NASV - See how great a love the Father has bestowed on us, that we would be called children of God; and such we are. For this reason the world does not know us, because it did not know Him.  (Aleph, A, B)

I John 3:5

KJV - And ye know that he was manifested to take away our sins; and in him is no sin. (Aleph, Byz)

NASV - You know that He appeared in order to take away sins; and in Him there is no sin. (A, B)

I John 3:13

KJV - Marvel not, my brethren, if the world hate you. (Byz)

NASV - Do not be surprised, brethren, if the world hates you. (non-Byzantine manuscripts)

I John 3:14

KJV - We know that we have passed from death unto life, because we love the brethren. He that loveth not his brother abideth in death. (Byz)

NASV - We know that we have passed out of death into life, because we love the brethren. He who does not love abides in death.  (Aleph, A, B)

I John 3:18

KJV - My little children, let us not love in word, neither in tongue; but in deed and in truth. (Byz)

NASV - Little children, let us not love with word or with tongue, but in deed and truth. (non-Byzantine manuscripts)

I John 3:19

KJV - And hereby we know that we are of the truth, and shall assure our hearts before him.  (Byz)

NASV - We will know by this that we are of the truth, and will assure our heart before Him.  (Aleph, A, B)

I John 4:3

KJV - And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world.  (Byz)

NASV - and every spirit that does not confess Jesus is not from God; this is the spirit of the antichrist, of which you have heard that it is coming, and now it is already in the world.  (A, B)

I John 4:19

KJV - We love him, because he first loved us.  (Byz)

NASV - We love, because He first loved us.  (A, B)

I John 4:20

KJV - If a man say, I love God, and hateth his brother, he is a liar: for he that loveth not his brother whom he hath seen, how can he love God whom he hath not seen?  (A, Byz)

NASV - If someone says, "I love God," and hates his brother, he is a liar; for the one who does not love his brother whom he has seen, (he) cannot love God whom he has not seen.  (Aleph, B)

I John 5:7-8

KJV - 7 For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. 8 And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one. (no major manuscripts)

NASV - 7 For there are three that testify: 8 the Spirit and the water and the blood; and the three are in agreement. (Aleph, A, B, Byz)

I John 5:13

KJV - These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God. (Byz)

NASV - These things I have written to you who believe in the name of the Son of God, so that you may know that you have eternal life. (non-Byzantine manuscripts)

Note: NASV sources read "These things I have written to you so that you may know that you have eternal life, who believe in the name of the Son of God."

I John 5:21

KJV - Little children, keep yourselves from idols. Amen.  (Byz)

NASV - Little children, guard yourselves from idols.  (Aleph, A, B)

II John 1:3

KJV - Grace be with you, mercy, and peace, from God the Father, and from the Lord Jesus Christ, the Son of the Father, in truth and love.  (Aleph, Byz)

NASV - Grace, mercy and peace will be with us, from God the Father and from Jesus Christ, the Son of the Father, in truth and love.  (A, B)

II John 1:7

KJV - For many deceivers are entered into the world, who confess not that Jesus Christ is come in the flesh. This is a deceiver and an antichrist. (Byz)

NASV - For many deceivers have gone out into the world, those who do not acknowledge Jesus Christ as coming in the flesh. This is the deceiver and the antichrist. (non-Byzantine manuscripts)

II John 1:8

KJV - Look to yourselves, that we lose not those things which we have wrought, but that we receive a full reward.  (Byz)

NASV - Watch yourselves, that you do not lose what we have accomplished, but that you may receive a full reward.  (Aleph, A, B)

II John 1:9

KJV - Whosoever transgresseth, and abideth not in the doctrine of Christ, hath not God. He that abideth in the doctrine of Christ, he hath both the Father and the Son.  (Byz)

NASV - Anyone who goes too far and does not abide in the teaching of Christ, does not have God; the one who abides in the teaching, he has both the Father and the Son.  (Aleph, A, B)

II John 1:12

KJV - Having many things to write unto you, I would not write with paper and ink: but I trust to come unto you, and speak face to face, that our joy may be full.  (Aleph, Byz)

NASV - Though I have many things to write to you, I do not want to do so with paper and ink; but I hope to come to you and speak face to face, so that your joy may be made full.  (A, B)

II John 1:13

KJV - The children of thy elect sister greet thee. Amen.  (Byz)

NASV - The children of your chosen sister greet you.  (Aleph, A, B)

III John 1:9

KJV - I wrote unto the church: but Diotrephes, who loveth to have the preeminence among them, receiveth us not.  (Byz)

NASV - I wrote something to the church; but Diotrephes, who loves to be first among them, does not accept what we say.  (Aleph, A)