The King James Version (1611) was translated from the Textus Receptus, also called the Received Text. This Greek New Testament text is based on the vast majority of surviving manuscripts — over 5,000 Greek manuscripts that agree with one another. These manuscripts were copied and preserved by faithful believers throughout the centuries across the Byzantine Empire.
Modern versions such as the NIV, ESV, and NASB are based on the Critical Text, which draws primarily from two Alexandrian manuscripts: Codex Vaticanus (found in the Vatican library) and Codex Sinaiticus (found in a trash can at a monastery). These two manuscripts disagree with each other in thousands of places and disagree with the vast majority of manuscripts. Despite this, modern textual critics treat these corrupted manuscripts as superior simply because they are older — ignoring the fact that they were likely preserved only because faithful churches rejected them as corrupt and stopped copying them.
The Textus Receptus represents the text that the church of God actually used, believed, and transmitted throughout history. The Critical Text represents manuscripts that were discarded and forgotten by the early church because of their corruptions.