There are several verses included in the King James Version that are not included in modern translations (e.g. NIV, ESV, etc.). The KJV translators used the best manuscripts available to them in 1611. For the time period in which it was released, the King James translation was a monumental achievement.
Since then, older manuscripts have been discovered and carefully evaluated by scholars with the conclusion that the older manuscripts are more reliable. This has given modern translators unprecedented access to manuscripts closer in time to the original documents. The missing verses in the KJV are not found in the oldest and most reliable manuscripts. As a result, newer translations do not include these verses, but do make reference to them in the footnotes.
It is important to note that no doctrines of the Christian faith are affected by the differences between the KJV and modern translations. We recommend reading through the introduction of the Bible you’re using for more information about the translators’ philosophy about issues like this.
You can find out more about the New International Version (NIV) and the way it was translated here: http://www.thenivbible.com/about-the-niv/