Hi! I've missed you is using the contraction I've (I have) and it's the present perfect...meaning it is ongoing. It began in the past and continues until now. Saying it this way offers more emphasis. If someone tells you, "I've missed you," it's deep, meaningful, and heartfelt. I've (I have) missed you all these years. I've (I have) missed your company. I've really missed you. We couldn't get anything done without you. I missed you can be equally as heartfelt, but it is in the past and does not necessarily include a feeling that is ongoing. I missed you is a more casual way of saying you miss someone. I missed you! I'm glad you're back. I missed you. Hopefully I'll catch you next time. Ultimately, you can say both and still be understood. A native speaker of English would use "I've missed you" for more serious, purposeful situations and serious relationships and "I missed you" for more casual speech or common relational expressions. Hope this helps! -Michelle R.

That depends primarily on the time scale. In the simple case, when there is no extra time information, and you can assume that it relates to time up until the present, there is little difference. The slight difference is that 'have been' gives an impression of a longer time, so is probably a nicer thing to say, if you just want to convey your feelings to someone. When there are some time scales, the situation changes. "I have missed you" relates to times up to the present. So, you can't use it for times that are in the past. e.g. 'I missed you at the party last night'. In that case, you can't use 'I have...'.