The first consumer high definition camcorder formats to arrive on the market were tape-based, like DV - hence the name HDV. The aim was to maintain compatibility with the existing tape mechanisms, tape formats, and FireWire-based data transfer, so these were all kept essentially the same.
Although you can't play HDV on a DV device, keeping everything but the video format identical has meant the same camcorder model can record DV as well as HDV. Despite HD having a lot more pixels per frame than SD, using MPEG-2 has meant the data rate can be kept approximately the same as DV, so a one-hour DV tape also fits an hour of HDV.
When JVC launched the first consumer high definition camcorders in Japan and the US, it opted for 720p as its video format. This was the first version of HDV, known as HDV1. But JVC's HD products never made it to Europe, so we had to wait for Sony's version. This uses the non-standard 1,440 x 1,080 variant of 1080i, and is called HDV2. The same format has also been adopted by Canon and Panasonic for their HDV models.
But, as we explained earlier, DV eats up too much storage for media other than tape to be practical, and so does HDV. As a result, camcorder manufacturers looked for an alternative compression method for DVD, hard drive and Flash memory camcorders. Not surprisingly, the searched ended with MPEG-4 AVC H.264, as it can halve the data rate over MPEG-2 for comparative video quality. The AVCHD format was born.
Camcorder Formats Explained - HDV