I've seen some where a shear fabric is draped up to a high center point to result in a triangle type display. With some relaxed curves in the fabric it looks pretty good.
There is no easy way to dress a Palladian Window. They make flexible hardware for draperies but you have to order them. The least expensive thing to do is fabric sheered on a flex rod. You can create one yourself using PVC pipe. Another idea is to hang a curtain straight across & leave the arc open but this will not give full privacy. I hope this helps.
Depending on the height of the window, you can leave it open or if you are looking for privacy spray a clear coat of poly and the. Apply a stencil of vine
Or your preference. Very inexpensive
a friend bought those paper looking horizontally pleated blinds. sh gathered them at the top and pinched the center and folded them down to fit the arch. so sorry i cannot explain better. its like one of those old fashioned fans.
Kelly O, that is a great idea, the paper shades are sold at Lowes, they are used as temporary shades and can be cut and installed with tape.
If you want it darker & cooler...do the paper fan method but buy 2...one for inside & 1 facing out...sandwich a cut out cardboard...will be good for 10 years or more...that's what I did:-)