Updated 11/29/2017: that minus value is motivating....
I have booted both "Clonezilla Live" and ESXi 5.5.0 from a USB thumb drive to system memory. Without a hard drive. The images for both are on the Internet, and they're both basically Ubuntu. But an ISO image doesn't contain a preamble that will load into memory or down to mass storage (hdd, ssd). PXE supports both "Legacy" and "uEFI" boots. If you can find a Legacy or uEFI bootable image on the internet that's configured to load and run strictly is memory, it should be possible for PXE to load that just like it loads Clonezilla in our lab. You might look on Clonezilla's website, or Linux distro web sites.
You might also consider the security implications of grabbing something off the internet and booting your computer with it. I have experience that leads me to trust Clonezilla and ESXi and Ubuntu and OpenSUSE images I've downloaded. Executables from other places? Mmmmmmm. Might be better to make a thumb-drive or connect a DVD/CD rom reader. Or set-up Clonezilla on another local machine.
I'm most familiar with PXE boot from Clonezilla. Our Clonezilla server is set-up to have to know the MAC of the client computer, in advance. It also creates an IP address for each MAC it knows.
The client's PXE gets on the network and announces itself, by MAC, asking if anyone has something for it. Clonezilla can tell the client to boot itself locally, or it can send an all-in-memory-Linux to the client and then use that to download a disk image (Win, Linux, Mac, whatever) that is then booted. By default, we have Clonezilla (server) tell PXE to boot from local storage and gives it the correct IP address. This is all done from the network interface, the CPU has no idea what's happening. It gets told to boot a local volume, it does, done.
If Clonezilla server tells it to reimage, PXE pulls down the first piece of the memory-resident Clonezilla client bootable, sticks it in the CPU's memory and causes the CPU to jump to it. In that first piece is enough for the CPU to get PXE and the network interface to fetch and down-load the at least one more block, and with those 2, it can get more, until it's eventually downloaded all of the memory resident Clonezilla client OS, installed it correctly in memory, and branched the CPU to running it.
Once Clonezilla client is running on the target machine, Clonezilla Server will send the disk image it was set-up to send, and when that disk image finishes loading, the CPU will boot from it.