Physical RAM is basically Random Access Memory that is attached to a Web Server. Whereas Virtual RAM is a concept when we speak about Virtual Private Servers or VPS Hosting packages. RAM is assigned to a VPS Hosting package through a Virtualization Software such as OpenVZ or Virtuozzo Power Panel which is powered by Parallels INC.
RAM is one of the most important concept in VPS. Physical RAM has no concept of Burstable RAM or Guaranteed RAM and it will only provide the same as it mentions, whereas Guaranteed RAM and Burstable RAM both are components of Virtual RAM.