You can't do "via" cloud. The cloud is present on both Xbox and WP. So say for example you purchase a song using your WP, it sync that song to cloud. Now when you turn on your Xbox "ideally" that song is in your collection (in the cloud). So you just hit play and it will play that song provided you are connected to WiFi to access the cloud.
I think when the bugs are ironed out, you can purchase song on WP, and stream that song from your collection in cloud, on your Xbox. Right now, downloading/purchasing songs from WP is hit and miss. Some songs sync to cloud, some don't.
In a nutshell, anything that correctly syncs to the cloud will be available on all three screens - Xbox, WP, Windows 8.
Right now, this is only true if the song is downloaded/purchased/added to collection on Windows 8 PC/laptop first. However, there are 50-50% chances, doing the same on WP, will sync correctly to cloud. But, with the state in which Xbox Music on WP is, I know it doesn't work every time.