I have a duvet and I think it’s great. It’s thick, warm and comfortable. I also like that when it gets a bit dirty, I can remove the duvet cover and throw it in the washing machine and the dryer. The only annoying part of it is that the duvet sometimes moves around a bit inside the cover. It bunches up in places, and the corners especially don’t seem to want to stay in place for very long. I only heard about duvet holders recently, and have not gotten any yet. But I did a bit of research and found that there are basically two kinds available. First is the duvet doughnut, which comes in two parts. The disc-like part goes on one side of the duvet cover, and the ring-like part goes on the other side, sort of snapping together with the duvet and the cover between the two parts of the donuts. They are made of see-through plastic, so they don’t clash with the color of the duvet cover.
Pictured: Set Of 4 Comforter Bed Duvet Donuts. Holder/ Gripper/ Fastener and Prevents Bunching & Shifting of Comforters , Blankets and Duvets. Keeps Corners in Place. Fits All Sizes/Types of Duvets and Comforters Without Harming Fabric
The other type are comforter cover clips, which look a little like big plastic binder clips. They are simpler to use: just clip the duvet in place so that it cannot slide inside its cover. Some are apparently foam covered, which is a good idea, since you may come into contact with them while sleeping. They are more visible than the donuts.
Pictured: Set Of 4 Padded Comforter Clips, Prevents Comforters From Shifting Inside Duvet Cover
These are not to be confused with quilt clips which, from what I can gather, are mainly used to hang a quilt on the wall. There is also a product called quilt clips which is made to be used only to make placing a new duvet cover easier. They hold the corners while you place the cover, and then are removed.