The NIV is different from the KJV because (1) it used older and potentially better manuscripts as a base and (2) the translators knew Hebrew better.
However the NIV still openly preserves consistency between the New and Old Testaments even at the expense of using translations that are recognized as being suspect.
The best example is Isaiah 7:14.
The NIV renders this verse as:
“Therefore the Lord himself will give you a sign: The virgin will be with child and will give birth to a son, and will call him Immanuel.”
This is consistent with Matthew 1:22-23.
“All this took place to fulfill what the Lord had said through the prophet: The virgin will be with child and will give birth to a son, and they will call him Immanuel”
The problem is that Matthew is quoting from the Greek Septuagint which does in fact use the Greek "parthenos" which specifically means virgin. In the original Hebrew however, the word is "almah," which some would argue simply means “young woman” and not necessarily a virgin. If Isaiah meant “virgin,” these folks point out, he would have used the Hebrew "betulah" which he uses elsewhere and which can only mean virgin. There are also some disagreements about tense.
The New Revised Standard Version (NRSV), claimed by some to be the most accurate English translation, and which doesn't concern itself with maintaining consistency with the New Testament, translates the passage as:
“Therefore the Lord himself will give you a sign. Look, the young woman is with child and shall bear a son, and shall name him Immanuel.”
In the NRSV the woman is already pregnant which sort of eliminates any question about her virginity.
