Suppose I want to run ubuntu on virtual box.. how it effect original windows in terms of speed ?? like we used ram for ubuntu can not be used for windows like that??
I would have to say that this is an open-ended question. It all depends of the processor speed, Amount of RAM, Graphics Adapter, and other resources.
To give you an idea, I have a laptop that has an Intel Core 2 Duo running at 202GHz, w/4GB RAM, and Windows 8 as the Host OS, and Ubuntu 13.04, Xubuntu 13.04, and Lubuntu 13.04 installed in a VM. Running one VM slows Windows about 20%, and running 2 VM's at the same time slows it down about 60%. (all VM's have 1GB RAM)
On the other hand, I have a laptop that has an Intel i5 running at 303GHz, w/4GB RAM Running a total of 10VM's and I start seeing slow down, after running 3 VM's at the same time (all VM's have 1GB RAM).
A Lenovo with i7 3.6GHz and 8GB RAM slowness happens after running 5 VM's at the same time.
Now keep in mind that my definition of slow and yours might be different.
A VM works just like any other program: if it spends a lot of time processing or chews up a lot of RAM, those resources aren't going to be available for other programs.
The only thing that makes VMs difficult to get a handle on — in terms of resource usage — is that inside the virtual machine program there are other programs. This nesting effect means that to work out the "cost" of the VM, you must consider the aggregate cost of everything that is running inside the VM.
If the VM is completely busy, it will take all of the resources you have assigned to that VM, just like any other completely busy program would. The only thing that makes a VM odd in this respect is that virtual machine systems let you specify how much CPU and RAM each VM is allowed to use.
If the VM is sitting there idle, it might effectively have zero impact on the host OS.
Partly, this is because modern OSes are good at signaling idleness to the underlying hardware — the virtual machine system in this case — in order to get good battery life on mobile systems. Thus, the virtual machine system doesn't spend much CPU time itself processing an idle guest OS.
An idle VM unavoidably takes some memory, but the host OS's virtual memory system should swap it out if another program comes along and needs some of the RAM the VM is using. That is to say, if you have 8 VMs running at a time, each configured for 2 GB of RAM, you might be able to get it to run quite well on a system with less than 16 GB of RAM. This is called over-provisioning.
I've only talked about RAM and disk usage above, but there are other resources to consider. Disk I/O, network bandwidth, etc. The ideas above extend to these shared resources just as well.
Disk I/O is worth thinking about specially, though. Because hard disks are relatively slow and have high seek times, a lot of VMs doing heavy disk I/O hurts overall I/O performance badly. SSDs and many-spindled RAIDs can help here, just as with any other high-disk-I/O problem.